Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

14 files changed, 6323 insertions, 0 deletions

diff --git a/servo/components/selectors/Cargo.toml b/servo/components/selectors/Cargo.toml
new file mode 100644
index 0000000000..2a8fe3a31e
--- /dev/null
+++ b/servo/components/selectors/Cargo.toml
@@ -0,0 +1,36 @@
+[package]
+name = "selectors"
+version = "0.22.0"
+authors = ["The Servo Project Developers"]
+documentation = "https://docs.rs/selectors/"
+description = "CSS Selectors matching for Rust"
+repository = "https://github.com/servo/servo"
+readme = "README.md"
+keywords = ["css", "selectors"]
+license = "MPL-2.0"
+build = "build.rs"
+
+[lib]
+name = "selectors"
+path = "lib.rs"
+
+[features]
+bench = []
+
+[dependencies]
+bitflags = "1.0"
+matches = "0.1"
+cssparser = "0.29"
+derive_more = { version = "0.99", default-features = false, features = ["add", "add_assign"] }
+fxhash = "0.2"
+log = "0.4"
+phf = "0.10"
+precomputed-hash = "0.1"
+servo_arc = { version = "0.1", path = "../servo_arc" }
+smallvec = "1.0"
+to_shmem = { path = "../to_shmem" }
+to_shmem_derive = { path = "../to_shmem_derive" }
+new_debug_unreachable = "1"
+
+[build-dependencies]
+phf_codegen = "0.10"
diff --git a/servo/components/selectors/README.md b/servo/components/selectors/README.md
new file mode 100644
index 0000000000..dac4a7ff91
--- /dev/null
+++ b/servo/components/selectors/README.md
@@ -0,0 +1,25 @@
+rust-selectors
+==============
+
+* [![Build Status](https://travis-ci.com/servo/rust-selectors.svg?branch=master)](
+  https://travis-ci.com/servo/rust-selectors)
+* [Documentation](https://docs.rs/selectors/)
+* [crates.io](https://crates.io/crates/selectors)
+
+CSS Selectors library for Rust.
+Includes parsing and serilization of selectors,
+as well as matching against a generic tree of elements.
+Pseudo-elements and most pseudo-classes are generic as well.
+
+**Warning:** breaking changes are made to this library fairly frequently
+(13 times in 2016, for example).
+However you can use this crate without updating it that often,
+old versions stay available on crates.io and Cargo will only automatically update
+to versions that are numbered as compatible.
+
+To see how to use this library with your own tree representation,
+see [Kuchiki’s `src/select.rs`](https://github.com/kuchiki-rs/kuchiki/blob/master/src/select.rs).
+(Note however that Kuchiki is not always up to date with the latest rust-selectors version,
+so that code may need to be tweaked.)
+If you don’t already have a tree data structure,
+consider using [Kuchiki](https://github.com/kuchiki-rs/kuchiki) itself.
diff --git a/servo/components/selectors/attr.rs b/servo/components/selectors/attr.rs
new file mode 100644
index 0000000000..9212f8c3a6
--- /dev/null
+++ b/servo/components/selectors/attr.rs
@@ -0,0 +1,188 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::parser::SelectorImpl;
+use cssparser::ToCss;
+use std::fmt;
+
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct AttrSelectorWithOptionalNamespace<Impl: SelectorImpl> {
+    #[shmem(field_bound)]
+    pub namespace: Option<NamespaceConstraint<(Impl::NamespacePrefix, Impl::NamespaceUrl)>>,
+    #[shmem(field_bound)]
+    pub local_name: Impl::LocalName,
+    pub local_name_lower: Impl::LocalName,
+    #[shmem(field_bound)]
+    pub operation: ParsedAttrSelectorOperation<Impl::AttrValue>,
+    pub never_matches: bool,
+}
+
+impl<Impl: SelectorImpl> AttrSelectorWithOptionalNamespace<Impl> {
+    pub fn namespace(&self) -> Option<NamespaceConstraint<&Impl::NamespaceUrl>> {
+        self.namespace.as_ref().map(|ns| match ns {
+            NamespaceConstraint::Any => NamespaceConstraint::Any,
+            NamespaceConstraint::Specific((_, ref url)) => NamespaceConstraint::Specific(url),
+        })
+    }
+}
+
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+pub enum NamespaceConstraint<NamespaceUrl> {
+    Any,
+
+    /// Empty string for no namespace
+    Specific(NamespaceUrl),
+}
+
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+pub enum ParsedAttrSelectorOperation<AttrValue> {
+    Exists,
+    WithValue {
+        operator: AttrSelectorOperator,
+        case_sensitivity: ParsedCaseSensitivity,
+        expected_value: AttrValue,
+    },
+}
+
+#[derive(Clone, Eq, PartialEq)]
+pub enum AttrSelectorOperation<AttrValue> {
+    Exists,
+    WithValue {
+        operator: AttrSelectorOperator,
+        case_sensitivity: CaseSensitivity,
+        expected_value: AttrValue,
+    },
+}
+
+impl<AttrValue> AttrSelectorOperation<AttrValue> {
+    pub fn eval_str(&self, element_attr_value: &str) -> bool
+    where
+        AttrValue: AsRef<str>,
+    {
+        match *self {
+            AttrSelectorOperation::Exists => true,
+            AttrSelectorOperation::WithValue {
+                operator,
+                case_sensitivity,
+                ref expected_value,
+            } => operator.eval_str(
+                element_attr_value,
+                expected_value.as_ref(),
+                case_sensitivity,
+            ),
+        }
+    }
+}
+
+#[derive(Clone, Copy, Eq, PartialEq, ToShmem)]
+pub enum AttrSelectorOperator {
+    Equal,
+    Includes,
+    DashMatch,
+    Prefix,
+    Substring,
+    Suffix,
+}
+
+impl ToCss for AttrSelectorOperator {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        // https://drafts.csswg.org/cssom/#serializing-selectors
+        // See "attribute selector".
+        dest.write_str(match *self {
+            AttrSelectorOperator::Equal => "=",
+            AttrSelectorOperator::Includes => "~=",
+            AttrSelectorOperator::DashMatch => "|=",
+            AttrSelectorOperator::Prefix => "^=",
+            AttrSelectorOperator::Substring => "*=",
+            AttrSelectorOperator::Suffix => "$=",
+        })
+    }
+}
+
+impl AttrSelectorOperator {
+    pub fn eval_str(
+        self,
+        element_attr_value: &str,
+        attr_selector_value: &str,
+        case_sensitivity: CaseSensitivity,
+    ) -> bool {
+        let e = element_attr_value.as_bytes();
+        let s = attr_selector_value.as_bytes();
+        let case = case_sensitivity;
+        match self {
+            AttrSelectorOperator::Equal => case.eq(e, s),
+            AttrSelectorOperator::Prefix => e.len() >= s.len() && case.eq(&e[..s.len()], s),
+            AttrSelectorOperator::Suffix => {
+                e.len() >= s.len() && case.eq(&e[(e.len() - s.len())..], s)
+            },
+            AttrSelectorOperator::Substring => {
+                case.contains(element_attr_value, attr_selector_value)
+            },
+            AttrSelectorOperator::Includes => element_attr_value
+                .split(SELECTOR_WHITESPACE)
+                .any(|part| case.eq(part.as_bytes(), s)),
+            AttrSelectorOperator::DashMatch => {
+                case.eq(e, s) || (e.get(s.len()) == Some(&b'-') && case.eq(&e[..s.len()], s))
+            },
+        }
+    }
+}
+
+/// The definition of whitespace per CSS Selectors Level 3 § 4.
+pub static SELECTOR_WHITESPACE: &[char] = &[' ', '\t', '\n', '\r', '\x0C'];
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, ToShmem)]
+pub enum ParsedCaseSensitivity {
+    /// 's' was specified.
+    ExplicitCaseSensitive,
+    /// 'i' was specified.
+    AsciiCaseInsensitive,
+    /// No flags were specified and HTML says this is a case-sensitive attribute.
+    CaseSensitive,
+    /// No flags were specified and HTML says this is a case-insensitive attribute.
+    AsciiCaseInsensitiveIfInHtmlElementInHtmlDocument,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum CaseSensitivity {
+    CaseSensitive,
+    AsciiCaseInsensitive,
+}
+
+impl CaseSensitivity {
+    pub fn eq(self, a: &[u8], b: &[u8]) -> bool {
+        match self {
+            CaseSensitivity::CaseSensitive => a == b,
+            CaseSensitivity::AsciiCaseInsensitive => a.eq_ignore_ascii_case(b),
+        }
+    }
+
+    pub fn contains(self, haystack: &str, needle: &str) -> bool {
+        match self {
+            CaseSensitivity::CaseSensitive => haystack.contains(needle),
+            CaseSensitivity::AsciiCaseInsensitive => {
+                if let Some((&n_first_byte, n_rest)) = needle.as_bytes().split_first() {
+                    haystack.bytes().enumerate().any(|(i, byte)| {
+                        if !byte.eq_ignore_ascii_case(&n_first_byte) {
+                            return false;
+                        }
+                        let after_this_byte = &haystack.as_bytes()[i + 1..];
+                        match after_this_byte.get(..n_rest.len()) {
+                            None => false,
+                            Some(haystack_slice) => haystack_slice.eq_ignore_ascii_case(n_rest),
+                        }
+                    })
+                } else {
+                    // any_str.contains("") == true,
+                    // though these cases should be handled with *NeverMatches and never go here.
+                    true
+                }
+            },
+        }
+    }
+}
diff --git a/servo/components/selectors/bloom.rs b/servo/components/selectors/bloom.rs
new file mode 100644
index 0000000000..98461d1ba2
--- /dev/null
+++ b/servo/components/selectors/bloom.rs
@@ -0,0 +1,422 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Counting and non-counting Bloom filters tuned for use as ancestor filters
+//! for selector matching.
+
+use std::fmt::{self, Debug};
+
+// The top 8 bits of the 32-bit hash value are not used by the bloom filter.
+// Consumers may rely on this to pack hashes more efficiently.
+pub const BLOOM_HASH_MASK: u32 = 0x00ffffff;
+const KEY_SIZE: usize = 12;
+
+const ARRAY_SIZE: usize = 1 << KEY_SIZE;
+const KEY_MASK: u32 = (1 << KEY_SIZE) - 1;
+
+/// A counting Bloom filter with 8-bit counters.
+pub type BloomFilter = CountingBloomFilter<BloomStorageU8>;
+
+/// A counting Bloom filter with parameterized storage to handle
+/// counters of different sizes.  For now we assume that having two hash
+/// functions is enough, but we may revisit that decision later.
+///
+/// The filter uses an array with 2**KeySize entries.
+///
+/// Assuming a well-distributed hash function, a Bloom filter with
+/// array size M containing N elements and
+/// using k hash function has expected false positive rate exactly
+///
+/// $  (1 - (1 - 1/M)^{kN})^k  $
+///
+/// because each array slot has a
+///
+/// $  (1 - 1/M)^{kN}  $
+///
+/// chance of being 0, and the expected false positive rate is the
+/// probability that all of the k hash functions will hit a nonzero
+/// slot.
+///
+/// For reasonable assumptions (M large, kN large, which should both
+/// hold if we're worried about false positives) about M and kN this
+/// becomes approximately
+///
+/// $$  (1 - \exp(-kN/M))^k   $$
+///
+/// For our special case of k == 2, that's $(1 - \exp(-2N/M))^2$,
+/// or in other words
+///
+/// $$    N/M = -0.5 * \ln(1 - \sqrt(r))   $$
+///
+/// where r is the false positive rate.  This can be used to compute
+/// the desired KeySize for a given load N and false positive rate r.
+///
+/// If N/M is assumed small, then the false positive rate can
+/// further be approximated as 4*N^2/M^2.  So increasing KeySize by
+/// 1, which doubles M, reduces the false positive rate by about a
+/// factor of 4, and a false positive rate of 1% corresponds to
+/// about M/N == 20.
+///
+/// What this means in practice is that for a few hundred keys using a
+/// KeySize of 12 gives false positive rates on the order of 0.25-4%.
+///
+/// Similarly, using a KeySize of 10 would lead to a 4% false
+/// positive rate for N == 100 and to quite bad false positive
+/// rates for larger N.
+#[derive(Clone, Default)]
+pub struct CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    storage: S,
+}
+
+impl<S> CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    /// Creates a new bloom filter.
+    #[inline]
+    pub fn new() -> Self {
+        Default::default()
+    }
+
+    #[inline]
+    pub fn clear(&mut self) {
+        self.storage = Default::default();
+    }
+
+    // Slow linear accessor to make sure the bloom filter is zeroed. This should
+    // never be used in release builds.
+    #[cfg(debug_assertions)]
+    pub fn is_zeroed(&self) -> bool {
+        self.storage.is_zeroed()
+    }
+
+    #[cfg(not(debug_assertions))]
+    pub fn is_zeroed(&self) -> bool {
+        unreachable!()
+    }
+
+    /// Inserts an item with a particular hash into the bloom filter.
+    #[inline]
+    pub fn insert_hash(&mut self, hash: u32) {
+        self.storage.adjust_first_slot(hash, true);
+        self.storage.adjust_second_slot(hash, true);
+    }
+
+    /// Removes an item with a particular hash from the bloom filter.
+    #[inline]
+    pub fn remove_hash(&mut self, hash: u32) {
+        self.storage.adjust_first_slot(hash, false);
+        self.storage.adjust_second_slot(hash, false);
+    }
+
+    /// Check whether the filter might contain an item with the given hash.
+    /// This can sometimes return true even if the item is not in the filter,
+    /// but will never return false for items that are actually in the filter.
+    #[inline]
+    pub fn might_contain_hash(&self, hash: u32) -> bool {
+        !self.storage.first_slot_is_empty(hash) && !self.storage.second_slot_is_empty(hash)
+    }
+}
+
+impl<S> Debug for CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut slots_used = 0;
+        for i in 0..ARRAY_SIZE {
+            if !self.storage.slot_is_empty(i) {
+                slots_used += 1;
+            }
+        }
+        write!(f, "BloomFilter({}/{})", slots_used, ARRAY_SIZE)
+    }
+}
+
+pub trait BloomStorage: Clone + Default {
+    fn slot_is_empty(&self, index: usize) -> bool;
+    fn adjust_slot(&mut self, index: usize, increment: bool);
+    fn is_zeroed(&self) -> bool;
+
+    #[inline]
+    fn first_slot_is_empty(&self, hash: u32) -> bool {
+        self.slot_is_empty(Self::first_slot_index(hash))
+    }
+
+    #[inline]
+    fn second_slot_is_empty(&self, hash: u32) -> bool {
+        self.slot_is_empty(Self::second_slot_index(hash))
+    }
+
+    #[inline]
+    fn adjust_first_slot(&mut self, hash: u32, increment: bool) {
+        self.adjust_slot(Self::first_slot_index(hash), increment)
+    }
+
+    #[inline]
+    fn adjust_second_slot(&mut self, hash: u32, increment: bool) {
+        self.adjust_slot(Self::second_slot_index(hash), increment)
+    }
+
+    #[inline]
+    fn first_slot_index(hash: u32) -> usize {
+        hash1(hash) as usize
+    }
+
+    #[inline]
+    fn second_slot_index(hash: u32) -> usize {
+        hash2(hash) as usize
+    }
+}
+
+/// Storage class for a CountingBloomFilter that has 8-bit counters.
+pub struct BloomStorageU8 {
+    counters: [u8; ARRAY_SIZE],
+}
+
+impl BloomStorage for BloomStorageU8 {
+    #[inline]
+    fn adjust_slot(&mut self, index: usize, increment: bool) {
+        let slot = &mut self.counters[index];
+        if *slot != 0xff {
+            // full
+            if increment {
+                *slot += 1;
+            } else {
+                *slot -= 1;
+            }
+        }
+    }
+
+    #[inline]
+    fn slot_is_empty(&self, index: usize) -> bool {
+        self.counters[index] == 0
+    }
+
+    #[inline]
+    fn is_zeroed(&self) -> bool {
+        self.counters.iter().all(|x| *x == 0)
+    }
+}
+
+impl Default for BloomStorageU8 {
+    fn default() -> Self {
+        BloomStorageU8 {
+            counters: [0; ARRAY_SIZE],
+        }
+    }
+}
+
+impl Clone for BloomStorageU8 {
+    fn clone(&self) -> Self {
+        BloomStorageU8 {
+            counters: self.counters,
+        }
+    }
+}
+
+/// Storage class for a CountingBloomFilter that has 1-bit counters.
+pub struct BloomStorageBool {
+    counters: [u8; ARRAY_SIZE / 8],
+}
+
+impl BloomStorage for BloomStorageBool {
+    #[inline]
+    fn adjust_slot(&mut self, index: usize, increment: bool) {
+        let bit = 1 << (index % 8);
+        let byte = &mut self.counters[index / 8];
+
+        // Since we have only one bit for storage, decrementing it
+        // should never do anything.  Assert against an accidental
+        // decrementing of a bit that was never set.
+        assert!(
+            increment || (*byte & bit) != 0,
+            "should not decrement if slot is already false"
+        );
+
+        if increment {
+            *byte |= bit;
+        }
+    }
+
+    #[inline]
+    fn slot_is_empty(&self, index: usize) -> bool {
+        let bit = 1 << (index % 8);
+        (self.counters[index / 8] & bit) == 0
+    }
+
+    #[inline]
+    fn is_zeroed(&self) -> bool {
+        self.counters.iter().all(|x| *x == 0)
+    }
+}
+
+impl Default for BloomStorageBool {
+    fn default() -> Self {
+        BloomStorageBool {
+            counters: [0; ARRAY_SIZE / 8],
+        }
+    }
+}
+
+impl Clone for BloomStorageBool {
+    fn clone(&self) -> Self {
+        BloomStorageBool {
+            counters: self.counters,
+        }
+    }
+}
+
+#[inline]
+fn hash1(hash: u32) -> u32 {
+    hash & KEY_MASK
+}
+
+#[inline]
+fn hash2(hash: u32) -> u32 {
+    (hash >> KEY_SIZE) & KEY_MASK
+}
+
+#[test]
+fn create_and_insert_some_stuff() {
+    use fxhash::FxHasher;
+    use std::hash::{Hash, Hasher};
+    use std::mem::transmute;
+
+    fn hash_as_str(i: usize) -> u32 {
+        let mut hasher = FxHasher::default();
+        let s = i.to_string();
+        s.hash(&mut hasher);
+        let hash: u64 = hasher.finish();
+        (hash >> 32) as u32 ^ (hash as u32)
+    }
+
+    let mut bf = BloomFilter::new();
+
+    // Statically assert that ARRAY_SIZE is a multiple of 8, which
+    // BloomStorageBool relies on.
+    unsafe {
+        transmute::<[u8; ARRAY_SIZE % 8], [u8; 0]>([]);
+    }
+
+    for i in 0_usize..1000 {
+        bf.insert_hash(hash_as_str(i));
+    }
+
+    for i in 0_usize..1000 {
+        assert!(bf.might_contain_hash(hash_as_str(i)));
+    }
+
+    let false_positives = (1001_usize..2000)
+        .filter(|i| bf.might_contain_hash(hash_as_str(*i)))
+        .count();
+
+    assert!(false_positives < 190, "{} is not < 190", false_positives); // 19%.
+
+    for i in 0_usize..100 {
+        bf.remove_hash(hash_as_str(i));
+    }
+
+    for i in 100_usize..1000 {
+        assert!(bf.might_contain_hash(hash_as_str(i)));
+    }
+
+    let false_positives = (0_usize..100)
+        .filter(|i| bf.might_contain_hash(hash_as_str(*i)))
+        .count();
+
+    assert!(false_positives < 20, "{} is not < 20", false_positives); // 20%.
+
+    bf.clear();
+
+    for i in 0_usize..2000 {
+        assert!(!bf.might_contain_hash(hash_as_str(i)));
+    }
+}
+
+#[cfg(feature = "bench")]
+#[cfg(test)]
+mod bench {
+    extern crate test;
+    use super::BloomFilter;
+
+    #[derive(Default)]
+    struct HashGenerator(u32);
+
+    impl HashGenerator {
+        fn next(&mut self) -> u32 {
+            // Each hash is split into two twelve-bit segments, which are used
+            // as an index into an array. We increment each by 64 so that we
+            // hit the next cache line, and then another 1 so that our wrapping
+            // behavior leads us to different entries each time.
+            //
+            // Trying to simulate cold caches is rather difficult with the cargo
+            // benchmarking setup, so it may all be moot depending on the number
+            // of iterations that end up being run. But we might as well.
+            self.0 += (65) + (65 << super::KEY_SIZE);
+            self.0
+        }
+    }
+
+    #[bench]
+    fn create_insert_1000_remove_100_lookup_100(b: &mut test::Bencher) {
+        b.iter(|| {
+            let mut gen1 = HashGenerator::default();
+            let mut gen2 = HashGenerator::default();
+            let mut bf = BloomFilter::new();
+            for _ in 0_usize..1000 {
+                bf.insert_hash(gen1.next());
+            }
+            for _ in 0_usize..100 {
+                bf.remove_hash(gen2.next());
+            }
+            for _ in 100_usize..200 {
+                test::black_box(bf.might_contain_hash(gen2.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn might_contain_10(b: &mut test::Bencher) {
+        let bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        b.iter(|| {
+            for _ in 0..10 {
+                test::black_box(bf.might_contain_hash(gen.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn clear(b: &mut test::Bencher) {
+        let mut bf = Box::new(BloomFilter::new());
+        b.iter(|| test::black_box(&mut bf).clear());
+    }
+
+    #[bench]
+    fn insert_10(b: &mut test::Bencher) {
+        let mut bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        b.iter(|| {
+            for _ in 0..10 {
+                test::black_box(bf.insert_hash(gen.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn remove_10(b: &mut test::Bencher) {
+        let mut bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        // Note: this will underflow, and that's ok.
+        b.iter(|| {
+            for _ in 0..10 {
+                bf.remove_hash(gen.next())
+            }
+        });
+    }
+}
diff --git a/servo/components/selectors/build.rs b/servo/components/selectors/build.rs
new file mode 100644
index 0000000000..c5c3803991
--- /dev/null
+++ b/servo/components/selectors/build.rs
@@ -0,0 +1,77 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+extern crate phf_codegen;
+
+use std::env;
+use std::fs::File;
+use std::io::{BufWriter, Write};
+use std::path::Path;
+
+fn main() {
+    let path = Path::new(&env::var_os("OUT_DIR").unwrap())
+        .join("ascii_case_insensitive_html_attributes.rs");
+    let mut file = BufWriter::new(File::create(&path).unwrap());
+
+    let mut set = phf_codegen::Set::new();
+    for name in ASCII_CASE_INSENSITIVE_HTML_ATTRIBUTES.split_whitespace() {
+        set.entry(name);
+    }
+    write!(
+        &mut file,
+        "{{ static SET: ::phf::Set<&'static str> = {}; &SET }}",
+        set.build(),
+    )
+    .unwrap();
+}
+
+/// <https://html.spec.whatwg.org/multipage/#selectors>
+static ASCII_CASE_INSENSITIVE_HTML_ATTRIBUTES: &str = r#"
+    accept
+    accept-charset
+    align
+    alink
+    axis
+    bgcolor
+    charset
+    checked
+    clear
+    codetype
+    color
+    compact
+    declare
+    defer
+    dir
+    direction
+    disabled
+    enctype
+    face
+    frame
+    hreflang
+    http-equiv
+    lang
+    language
+    link
+    media
+    method
+    multiple
+    nohref
+    noresize
+    noshade
+    nowrap
+    readonly
+    rel
+    rev
+    rules
+    scope
+    scrolling
+    selected
+    shape
+    target
+    text
+    type
+    valign
+    valuetype
+    vlink
+"#;
diff --git a/servo/components/selectors/builder.rs b/servo/components/selectors/builder.rs
new file mode 100644
index 0000000000..044db14db1
--- /dev/null
+++ b/servo/components/selectors/builder.rs
@@ -0,0 +1,364 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Helper module to build up a selector safely and efficiently.
+//!
+//! Our selector representation is designed to optimize matching, and has
+//! several requirements:
+//! * All simple selectors and combinators are stored inline in the same buffer
+//!   as Component instances.
+//! * We store the top-level compound selectors from right to left, i.e. in
+//!   matching order.
+//! * We store the simple selectors for each combinator from left to right, so
+//!   that we match the cheaper simple selectors first.
+//!
+//! Meeting all these constraints without extra memmove traffic during parsing
+//! is non-trivial. This module encapsulates those details and presents an
+//! easy-to-use API for the parser.
+
+use crate::parser::{Combinator, Component, Selector, SelectorImpl};
+use crate::sink::Push;
+use servo_arc::{Arc, HeaderWithLength, ThinArc};
+use smallvec::{self, SmallVec};
+use std::cmp;
+use std::iter;
+use std::ptr;
+use std::slice;
+
+/// Top-level SelectorBuilder struct. This should be stack-allocated by the
+/// consumer and never moved (because it contains a lot of inline data that
+/// would be slow to memmov).
+///
+/// After instantation, callers may call the push_simple_selector() and
+/// push_combinator() methods to append selector data as it is encountered
+/// (from left to right). Once the process is complete, callers should invoke
+/// build(), which transforms the contents of the SelectorBuilder into a heap-
+/// allocated Selector and leaves the builder in a drained state.
+#[derive(Debug)]
+pub struct SelectorBuilder<Impl: SelectorImpl> {
+    /// The entire sequence of simple selectors, from left to right, without combinators.
+    ///
+    /// We make this large because the result of parsing a selector is fed into a new
+    /// Arc-ed allocation, so any spilled vec would be a wasted allocation. Also,
+    /// Components are large enough that we don't have much cache locality benefit
+    /// from reserving stack space for fewer of them.
+    simple_selectors: SmallVec<[Component<Impl>; 32]>,
+    /// The combinators, and the length of the compound selector to their left.
+    combinators: SmallVec<[(Combinator, usize); 16]>,
+    /// The length of the current compount selector.
+    current_len: usize,
+}
+
+impl<Impl: SelectorImpl> Default for SelectorBuilder<Impl> {
+    #[inline(always)]
+    fn default() -> Self {
+        SelectorBuilder {
+            simple_selectors: SmallVec::new(),
+            combinators: SmallVec::new(),
+            current_len: 0,
+        }
+    }
+}
+
+impl<Impl: SelectorImpl> Push<Component<Impl>> for SelectorBuilder<Impl> {
+    fn push(&mut self, value: Component<Impl>) {
+        self.push_simple_selector(value);
+    }
+}
+
+impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
+    /// Pushes a simple selector onto the current compound selector.
+    #[inline(always)]
+    pub fn push_simple_selector(&mut self, ss: Component<Impl>) {
+        assert!(!ss.is_combinator());
+        self.simple_selectors.push(ss);
+        self.current_len += 1;
+    }
+
+    /// Completes the current compound selector and starts a new one, delimited
+    /// by the given combinator.
+    #[inline(always)]
+    pub fn push_combinator(&mut self, c: Combinator) {
+        self.combinators.push((c, self.current_len));
+        self.current_len = 0;
+    }
+
+    /// Returns true if combinators have ever been pushed to this builder.
+    #[inline(always)]
+    pub fn has_combinators(&self) -> bool {
+        !self.combinators.is_empty()
+    }
+
+    /// Consumes the builder, producing a Selector.
+    #[inline(always)]
+    pub fn build(
+        &mut self,
+        parsed_pseudo: bool,
+        parsed_slotted: bool,
+        parsed_part: bool,
+    ) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // Compute the specificity and flags.
+        let specificity = specificity(self.simple_selectors.iter());
+        let mut flags = SelectorFlags::empty();
+        if parsed_pseudo {
+            flags |= SelectorFlags::HAS_PSEUDO;
+        }
+        if parsed_slotted {
+            flags |= SelectorFlags::HAS_SLOTTED;
+        }
+        if parsed_part {
+            flags |= SelectorFlags::HAS_PART;
+        }
+        self.build_with_specificity_and_flags(SpecificityAndFlags { specificity, flags })
+    }
+
+    /// Builds with an explicit SpecificityAndFlags. This is separated from build() so
+    /// that unit tests can pass an explicit specificity.
+    #[inline(always)]
+    pub fn build_with_specificity_and_flags(
+        &mut self,
+        spec: SpecificityAndFlags,
+    ) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // First, compute the total number of Components we'll need to allocate
+        // space for.
+        let full_len = self.simple_selectors.len() + self.combinators.len();
+
+        // Create the header.
+        let header = HeaderWithLength::new(spec, full_len);
+
+        // Create the Arc using an iterator that drains our buffers.
+
+        // Use a raw pointer to be able to call set_len despite "borrowing" the slice.
+        // This is similar to SmallVec::drain, but we use a slice here because
+        // we’re gonna traverse it non-linearly.
+        let raw_simple_selectors: *const [Component<Impl>] = &*self.simple_selectors;
+        unsafe {
+            // Panic-safety: if SelectorBuilderIter is not iterated to the end,
+            // some simple selectors will safely leak.
+            self.simple_selectors.set_len(0)
+        }
+        let (rest, current) = split_from_end(unsafe { &*raw_simple_selectors }, self.current_len);
+        let iter = SelectorBuilderIter {
+            current_simple_selectors: current.iter(),
+            rest_of_simple_selectors: rest,
+            combinators: self.combinators.drain(..).rev(),
+        };
+
+        Arc::into_thin(Arc::from_header_and_iter(header, iter))
+    }
+}
+
+struct SelectorBuilderIter<'a, Impl: SelectorImpl> {
+    current_simple_selectors: slice::Iter<'a, Component<Impl>>,
+    rest_of_simple_selectors: &'a [Component<Impl>],
+    combinators: iter::Rev<smallvec::Drain<'a, [(Combinator, usize); 16]>>,
+}
+
+impl<'a, Impl: SelectorImpl> ExactSizeIterator for SelectorBuilderIter<'a, Impl> {
+    fn len(&self) -> usize {
+        self.current_simple_selectors.len() +
+            self.rest_of_simple_selectors.len() +
+            self.combinators.len()
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for SelectorBuilderIter<'a, Impl> {
+    type Item = Component<Impl>;
+    #[inline(always)]
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(simple_selector_ref) = self.current_simple_selectors.next() {
+            // Move a simple selector out of this slice iterator.
+            // This is safe because we’ve called SmallVec::set_len(0) above,
+            // so SmallVec::drop won’t drop this simple selector.
+            unsafe { Some(ptr::read(simple_selector_ref)) }
+        } else {
+            self.combinators.next().map(|(combinator, len)| {
+                let (rest, current) = split_from_end(self.rest_of_simple_selectors, len);
+                self.rest_of_simple_selectors = rest;
+                self.current_simple_selectors = current.iter();
+                Component::Combinator(combinator)
+            })
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.len(), Some(self.len()))
+    }
+}
+
+fn split_from_end<T>(s: &[T], at: usize) -> (&[T], &[T]) {
+    s.split_at(s.len() - at)
+}
+
+bitflags! {
+    /// Flags that indicate at which point of parsing a selector are we.
+    #[derive(Default, ToShmem)]
+    pub (crate) struct SelectorFlags : u8 {
+        const HAS_PSEUDO = 1 << 0;
+        const HAS_SLOTTED = 1 << 1;
+        const HAS_PART = 1 << 2;
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, ToShmem)]
+pub struct SpecificityAndFlags {
+    /// There are two free bits here, since we use ten bits for each specificity
+    /// kind (id, class, element).
+    pub(crate) specificity: u32,
+    /// There's padding after this field due to the size of the flags.
+    pub(crate) flags: SelectorFlags,
+}
+
+impl SpecificityAndFlags {
+    #[inline]
+    pub fn specificity(&self) -> u32 {
+        self.specificity
+    }
+
+    #[inline]
+    pub fn has_pseudo_element(&self) -> bool {
+        self.flags.intersects(SelectorFlags::HAS_PSEUDO)
+    }
+
+    #[inline]
+    pub fn is_slotted(&self) -> bool {
+        self.flags.intersects(SelectorFlags::HAS_SLOTTED)
+    }
+
+    #[inline]
+    pub fn is_part(&self) -> bool {
+        self.flags.intersects(SelectorFlags::HAS_PART)
+    }
+}
+
+const MAX_10BIT: u32 = (1u32 << 10) - 1;
+
+#[derive(Add, AddAssign, Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)]
+struct Specificity {
+    id_selectors: u32,
+    class_like_selectors: u32,
+    element_selectors: u32,
+}
+
+impl From<u32> for Specificity {
+    #[inline]
+    fn from(value: u32) -> Specificity {
+        assert!(value <= MAX_10BIT << 20 | MAX_10BIT << 10 | MAX_10BIT);
+        Specificity {
+            id_selectors: value >> 20,
+            class_like_selectors: (value >> 10) & MAX_10BIT,
+            element_selectors: value & MAX_10BIT,
+        }
+    }
+}
+
+impl From<Specificity> for u32 {
+    #[inline]
+    fn from(specificity: Specificity) -> u32 {
+        cmp::min(specificity.id_selectors, MAX_10BIT) << 20 |
+            cmp::min(specificity.class_like_selectors, MAX_10BIT) << 10 |
+            cmp::min(specificity.element_selectors, MAX_10BIT)
+    }
+}
+
+fn specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> u32
+where
+    Impl: SelectorImpl,
+{
+    complex_selector_specificity(iter).into()
+}
+
+fn complex_selector_specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> Specificity
+where
+    Impl: SelectorImpl,
+{
+    fn simple_selector_specificity<Impl>(
+        simple_selector: &Component<Impl>,
+        specificity: &mut Specificity,
+    ) where
+        Impl: SelectorImpl,
+    {
+        match *simple_selector {
+            Component::Combinator(..) => {
+                unreachable!("Found combinator in simple selectors vector?");
+            },
+            Component::Part(..) | Component::PseudoElement(..) | Component::LocalName(..) => {
+                specificity.element_selectors += 1
+            },
+            Component::Slotted(ref selector) => {
+                specificity.element_selectors += 1;
+                // Note that due to the way ::slotted works we only compete with
+                // other ::slotted rules, so the above rule doesn't really
+                // matter, but we do it still for consistency with other
+                // pseudo-elements.
+                //
+                // See: https://github.com/w3c/csswg-drafts/issues/1915
+                *specificity += Specificity::from(selector.specificity());
+            },
+            Component::Host(ref selector) => {
+                specificity.class_like_selectors += 1;
+                if let Some(ref selector) = *selector {
+                    // See: https://github.com/w3c/csswg-drafts/issues/1915
+                    *specificity += Specificity::from(selector.specificity());
+                }
+            },
+            Component::ID(..) => {
+                specificity.id_selectors += 1;
+            },
+            Component::Class(..) |
+            Component::AttributeInNoNamespace { .. } |
+            Component::AttributeInNoNamespaceExists { .. } |
+            Component::AttributeOther(..) |
+            Component::Root |
+            Component::Empty |
+            Component::Scope |
+            Component::Nth(..) |
+            Component::NonTSPseudoClass(..) => {
+                specificity.class_like_selectors += 1;
+            },
+            Component::NthOf(ref nth_of_data) => {
+                // https://drafts.csswg.org/selectors/#specificity-rules:
+                //
+                //     The specificity of the :nth-last-child() pseudo-class,
+                //     like the :nth-child() pseudo-class, combines the
+                //     specificity of a regular pseudo-class with that of its
+                //     selector argument S.
+                specificity.class_like_selectors += 1;
+                *specificity += max_selector_list_specificity(nth_of_data.selectors());
+            },
+            Component::Negation(ref list) | Component::Is(ref list) | Component::Has(ref list) => {
+                // https://drafts.csswg.org/selectors/#specificity-rules:
+                //
+                //     The specificity of an :is(), :not(), or :has() pseudo-class
+                //     is replaced by the specificity of the most specific complex
+                //     selector in its selector list argument.
+                *specificity += max_selector_list_specificity(list);
+            },
+            Component::Where(..) |
+            Component::ExplicitUniversalType |
+            Component::ExplicitAnyNamespace |
+            Component::ExplicitNoNamespace |
+            Component::DefaultNamespace(..) |
+            Component::Namespace(..) => {
+                // Does not affect specificity
+            },
+        }
+    }
+
+    /// Finds the maximum specificity of elements in the list and returns it.
+    fn max_selector_list_specificity<Impl: SelectorImpl>(list: &[Selector<Impl>]) -> Specificity {
+        let max = list
+            .iter()
+            .map(|selector| selector.specificity())
+            .max()
+            .unwrap_or(0);
+        Specificity::from(max)
+    }
+
+    let mut specificity = Default::default();
+    for simple_selector in iter {
+        simple_selector_specificity(&simple_selector, &mut specificity);
+    }
+    specificity
+}
diff --git a/servo/components/selectors/context.rs b/servo/components/selectors/context.rs
new file mode 100644
index 0000000000..e29abe639a
--- /dev/null
+++ b/servo/components/selectors/context.rs
@@ -0,0 +1,310 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::attr::CaseSensitivity;
+use crate::bloom::BloomFilter;
+use crate::nth_index_cache::NthIndexCache;
+use crate::parser::SelectorImpl;
+use crate::tree::{Element, OpaqueElement};
+
+/// What kind of selector matching mode we should use.
+///
+/// There are two modes of selector matching. The difference is only noticeable
+/// in presence of pseudo-elements.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum MatchingMode {
+    /// Don't ignore any pseudo-element selectors.
+    Normal,
+
+    /// Ignores any stateless pseudo-element selectors in the rightmost sequence
+    /// of simple selectors.
+    ///
+    /// This is useful, for example, to match against ::before when you aren't a
+    /// pseudo-element yourself.
+    ///
+    /// For example, in presence of `::before:hover`, it would never match, but
+    /// `::before` would be ignored as in "matching".
+    ///
+    /// It's required for all the selectors you match using this mode to have a
+    /// pseudo-element.
+    ForStatelessPseudoElement,
+}
+
+/// The mode to use when matching unvisited and visited links.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum VisitedHandlingMode {
+    /// All links are matched as if they are unvisted.
+    AllLinksUnvisited,
+    /// All links are matched as if they are visited and unvisited (both :link
+    /// and :visited match).
+    ///
+    /// This is intended to be used from invalidation code, to be conservative
+    /// about whether we need to restyle a link.
+    AllLinksVisitedAndUnvisited,
+    /// A element's "relevant link" is the element being matched if it is a link
+    /// or the nearest ancestor link. The relevant link is matched as though it
+    /// is visited, and all other links are matched as if they are unvisited.
+    RelevantLinkVisited,
+}
+
+impl VisitedHandlingMode {
+    #[inline]
+    pub fn matches_visited(&self) -> bool {
+        matches!(
+            *self,
+            VisitedHandlingMode::RelevantLinkVisited |
+                VisitedHandlingMode::AllLinksVisitedAndUnvisited
+        )
+    }
+
+    #[inline]
+    pub fn matches_unvisited(&self) -> bool {
+        matches!(
+            *self,
+            VisitedHandlingMode::AllLinksUnvisited |
+                VisitedHandlingMode::AllLinksVisitedAndUnvisited
+        )
+    }
+}
+
+/// Whether we need to set selector invalidation flags on elements for this
+/// match request.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum NeedsSelectorFlags {
+    No,
+    Yes,
+}
+
+/// Which quirks mode is this document in.
+///
+/// See: https://quirks.spec.whatwg.org/
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub enum QuirksMode {
+    /// Quirks mode.
+    Quirks,
+    /// Limited quirks mode.
+    LimitedQuirks,
+    /// No quirks mode.
+    NoQuirks,
+}
+
+impl QuirksMode {
+    #[inline]
+    pub fn classes_and_ids_case_sensitivity(self) -> CaseSensitivity {
+        match self {
+            QuirksMode::NoQuirks | QuirksMode::LimitedQuirks => CaseSensitivity::CaseSensitive,
+            QuirksMode::Quirks => CaseSensitivity::AsciiCaseInsensitive,
+        }
+    }
+}
+
+/// Data associated with the matching process for a element.  This context is
+/// used across many selectors for an element, so it's not appropriate for
+/// transient data that applies to only a single selector.
+pub struct MatchingContext<'a, Impl>
+where
+    Impl: SelectorImpl,
+{
+    /// Input with the matching mode we should use when matching selectors.
+    matching_mode: MatchingMode,
+    /// Input with the bloom filter used to fast-reject selectors.
+    pub bloom_filter: Option<&'a BloomFilter>,
+    /// An optional cache to speed up nth-index-like selectors.
+    pub nth_index_cache: Option<&'a mut NthIndexCache>,
+    /// The element which is going to match :scope pseudo-class. It can be
+    /// either one :scope element, or the scoping element.
+    ///
+    /// Note that, although in theory there can be multiple :scope elements,
+    /// in current specs, at most one is specified, and when there is one,
+    /// scoping element is not relevant anymore, so we use a single field for
+    /// them.
+    ///
+    /// When this is None, :scope will match the root element.
+    ///
+    /// See https://drafts.csswg.org/selectors-4/#scope-pseudo
+    pub scope_element: Option<OpaqueElement>,
+
+    /// The current shadow host we're collecting :host rules for.
+    pub current_host: Option<OpaqueElement>,
+
+    /// Controls how matching for links is handled.
+    visited_handling: VisitedHandlingMode,
+
+    /// The current nesting level of selectors that we're matching.
+    ///
+    /// FIXME(emilio): Consider putting the mutable stuff in a Cell, then make
+    /// MatchingContext immutable again.
+    nesting_level: usize,
+
+    /// Whether we're inside a negation or not.
+    in_negation: bool,
+
+    /// An optional hook function for checking whether a pseudo-element
+    /// should match when matching_mode is ForStatelessPseudoElement.
+    pub pseudo_element_matching_fn: Option<&'a dyn Fn(&Impl::PseudoElement) -> bool>,
+
+    /// Extra implementation-dependent matching data.
+    pub extra_data: Impl::ExtraMatchingData<'a>,
+
+    quirks_mode: QuirksMode,
+    needs_selector_flags: NeedsSelectorFlags,
+    classes_and_ids_case_sensitivity: CaseSensitivity,
+    _impl: ::std::marker::PhantomData<Impl>,
+}
+
+impl<'a, Impl> MatchingContext<'a, Impl>
+where
+    Impl: SelectorImpl,
+{
+    /// Constructs a new `MatchingContext`.
+    pub fn new(
+        matching_mode: MatchingMode,
+        bloom_filter: Option<&'a BloomFilter>,
+        nth_index_cache: Option<&'a mut NthIndexCache>,
+        quirks_mode: QuirksMode,
+        needs_selector_flags: NeedsSelectorFlags,
+    ) -> Self {
+        Self::new_for_visited(
+            matching_mode,
+            bloom_filter,
+            nth_index_cache,
+            VisitedHandlingMode::AllLinksUnvisited,
+            quirks_mode,
+            needs_selector_flags,
+        )
+    }
+
+    /// Constructs a new `MatchingContext` for use in visited matching.
+    pub fn new_for_visited(
+        matching_mode: MatchingMode,
+        bloom_filter: Option<&'a BloomFilter>,
+        nth_index_cache: Option<&'a mut NthIndexCache>,
+        visited_handling: VisitedHandlingMode,
+        quirks_mode: QuirksMode,
+        needs_selector_flags: NeedsSelectorFlags,
+    ) -> Self {
+        Self {
+            matching_mode,
+            bloom_filter,
+            visited_handling,
+            nth_index_cache,
+            quirks_mode,
+            classes_and_ids_case_sensitivity: quirks_mode.classes_and_ids_case_sensitivity(),
+            needs_selector_flags,
+            scope_element: None,
+            current_host: None,
+            nesting_level: 0,
+            in_negation: false,
+            pseudo_element_matching_fn: None,
+            extra_data: Default::default(),
+            _impl: ::std::marker::PhantomData,
+        }
+    }
+
+    /// Whether we're matching a nested selector.
+    #[inline]
+    pub fn is_nested(&self) -> bool {
+        self.nesting_level != 0
+    }
+
+    /// Whether we're matching inside a :not(..) selector.
+    #[inline]
+    pub fn in_negation(&self) -> bool {
+        self.in_negation
+    }
+
+    /// The quirks mode of the document.
+    #[inline]
+    pub fn quirks_mode(&self) -> QuirksMode {
+        self.quirks_mode
+    }
+
+    /// The matching-mode for this selector-matching operation.
+    #[inline]
+    pub fn matching_mode(&self) -> MatchingMode {
+        self.matching_mode
+    }
+
+    /// Whether we need to set selector flags.
+    #[inline]
+    pub fn needs_selector_flags(&self) -> bool {
+        self.needs_selector_flags == NeedsSelectorFlags::Yes
+    }
+
+    /// The case-sensitivity for class and ID selectors
+    #[inline]
+    pub fn classes_and_ids_case_sensitivity(&self) -> CaseSensitivity {
+        self.classes_and_ids_case_sensitivity
+    }
+
+    /// Runs F with a deeper nesting level.
+    #[inline]
+    pub fn nest<F, R>(&mut self, f: F) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        self.nesting_level += 1;
+        let result = f(self);
+        self.nesting_level -= 1;
+        result
+    }
+
+    /// Runs F with a deeper nesting level, and marking ourselves in a negation,
+    /// for a :not(..) selector, for example.
+    #[inline]
+    pub fn nest_for_negation<F, R>(&mut self, f: F) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        let old_in_negation = self.in_negation;
+        self.in_negation = true;
+        let result = self.nest(f);
+        self.in_negation = old_in_negation;
+        result
+    }
+
+    #[inline]
+    pub fn visited_handling(&self) -> VisitedHandlingMode {
+        self.visited_handling
+    }
+
+    /// Runs F with a different VisitedHandlingMode.
+    #[inline]
+    pub fn with_visited_handling_mode<F, R>(
+        &mut self,
+        handling_mode: VisitedHandlingMode,
+        f: F,
+    ) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        let original_handling_mode = self.visited_handling;
+        self.visited_handling = handling_mode;
+        let result = f(self);
+        self.visited_handling = original_handling_mode;
+        result
+    }
+
+    /// Runs F with a given shadow host which is the root of the tree whose
+    /// rules we're matching.
+    #[inline]
+    pub fn with_shadow_host<F, E, R>(&mut self, host: Option<E>, f: F) -> R
+    where
+        E: Element,
+        F: FnOnce(&mut Self) -> R,
+    {
+        let original_host = self.current_host.take();
+        self.current_host = host.map(|h| h.opaque());
+        let result = f(self);
+        self.current_host = original_host;
+        result
+    }
+
+    /// Returns the current shadow host whose shadow root we're matching rules
+    /// against.
+    #[inline]
+    pub fn shadow_host(&self) -> Option<OpaqueElement> {
+        self.current_host
+    }
+}
diff --git a/servo/components/selectors/lib.rs b/servo/components/selectors/lib.rs
new file mode 100644
index 0000000000..53dbc8c536
--- /dev/null
+++ b/servo/components/selectors/lib.rs
@@ -0,0 +1,42 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// Make |cargo bench| work.
+#![cfg_attr(feature = "bench", feature(test))]
+
+#[macro_use]
+extern crate bitflags;
+#[macro_use]
+extern crate cssparser;
+#[macro_use]
+extern crate debug_unreachable;
+#[macro_use]
+extern crate derive_more;
+extern crate fxhash;
+#[macro_use]
+extern crate log;
+#[macro_use]
+extern crate matches;
+extern crate phf;
+extern crate precomputed_hash;
+extern crate servo_arc;
+extern crate smallvec;
+extern crate to_shmem;
+#[macro_use]
+extern crate to_shmem_derive;
+
+pub mod attr;
+pub mod bloom;
+mod builder;
+pub mod context;
+pub mod matching;
+mod nth_index_cache;
+pub mod parser;
+pub mod sink;
+mod tree;
+pub mod visitor;
+
+pub use crate::nth_index_cache::NthIndexCache;
+pub use crate::parser::{Parser, SelectorImpl, SelectorList};
+pub use crate::tree::{Element, OpaqueElement};
diff --git a/servo/components/selectors/matching.rs b/servo/components/selectors/matching.rs
new file mode 100644
index 0000000000..75eee961a2
--- /dev/null
+++ b/servo/components/selectors/matching.rs
@@ -0,0 +1,1001 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::attr::{
+    AttrSelectorOperation, CaseSensitivity, NamespaceConstraint, ParsedAttrSelectorOperation,
+    ParsedCaseSensitivity,
+};
+use crate::bloom::{BloomFilter, BLOOM_HASH_MASK};
+use crate::nth_index_cache::NthIndexCacheInner;
+use crate::parser::{AncestorHashes, Combinator, Component, LocalName, NthSelectorData};
+use crate::parser::{NonTSPseudoClass, Selector, SelectorImpl, SelectorIter, SelectorList};
+use crate::tree::Element;
+use smallvec::SmallVec;
+use std::borrow::Borrow;
+use std::iter;
+
+pub use crate::context::*;
+
+// The bloom filter for descendant CSS selectors will have a <1% false
+// positive rate until it has this many selectors in it, then it will
+// rapidly increase.
+pub static RECOMMENDED_SELECTOR_BLOOM_FILTER_SIZE: usize = 4096;
+
+bitflags! {
+    /// Set of flags that are set on either the element or its parent (depending
+    /// on the flag) if the element could potentially match a selector.
+    pub struct ElementSelectorFlags: usize {
+        /// When a child is added or removed from the parent, all the children
+        /// must be restyled, because they may match :nth-last-child,
+        /// :last-of-type, :nth-last-of-type, or :only-of-type.
+        const HAS_SLOW_SELECTOR = 1 << 0;
+
+        /// When a child is added or removed from the parent, any later
+        /// children must be restyled, because they may match :nth-child,
+        /// :first-of-type, or :nth-of-type.
+        const HAS_SLOW_SELECTOR_LATER_SIBLINGS = 1 << 1;
+
+        /// When a child is added or removed from the parent, the first and
+        /// last children must be restyled, because they may match :first-child,
+        /// :last-child, or :only-child.
+        const HAS_EDGE_CHILD_SELECTOR = 1 << 2;
+
+        /// The element has an empty selector, so when a child is appended we
+        /// might need to restyle the parent completely.
+        const HAS_EMPTY_SELECTOR = 1 << 3;
+    }
+}
+
+impl ElementSelectorFlags {
+    /// Returns the subset of flags that apply to the element.
+    pub fn for_self(self) -> ElementSelectorFlags {
+        self & (ElementSelectorFlags::HAS_EMPTY_SELECTOR)
+    }
+
+    /// Returns the subset of flags that apply to the parent.
+    pub fn for_parent(self) -> ElementSelectorFlags {
+        self & (ElementSelectorFlags::HAS_SLOW_SELECTOR |
+            ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS |
+            ElementSelectorFlags::HAS_EDGE_CHILD_SELECTOR)
+    }
+}
+
+/// Holds per-compound-selector data.
+struct LocalMatchingContext<'a, 'b: 'a, Impl: SelectorImpl> {
+    shared: &'a mut MatchingContext<'b, Impl>,
+    quirks_data: Option<(Rightmost, SelectorIter<'a, Impl>)>,
+}
+
+#[inline(always)]
+pub fn matches_selector_list<E>(
+    selector_list: &SelectorList<E::Impl>,
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+) -> bool
+where
+    E: Element,
+{
+    // This is pretty much any(..) but manually inlined because the compiler
+    // refuses to do so from querySelector / querySelectorAll.
+    for selector in &selector_list.0 {
+        let matches = matches_selector(selector, 0, None, element, context);
+        if matches {
+            return true;
+        }
+    }
+
+    false
+}
+
+#[inline(always)]
+fn may_match(hashes: &AncestorHashes, bf: &BloomFilter) -> bool {
+    // Check the first three hashes. Note that we can check for zero before
+    // masking off the high bits, since if any of the first three hashes is
+    // zero the fourth will be as well. We also take care to avoid the
+    // special-case complexity of the fourth hash until we actually reach it,
+    // because we usually don't.
+    //
+    // To be clear: this is all extremely hot.
+    for i in 0..3 {
+        let packed = hashes.packed_hashes[i];
+        if packed == 0 {
+            // No more hashes left - unable to fast-reject.
+            return true;
+        }
+
+        if !bf.might_contain_hash(packed & BLOOM_HASH_MASK) {
+            // Hooray! We fast-rejected on this hash.
+            return false;
+        }
+    }
+
+    // Now do the slighty-more-complex work of synthesizing the fourth hash,
+    // and check it against the filter if it exists.
+    let fourth = hashes.fourth_hash();
+    fourth == 0 || bf.might_contain_hash(fourth)
+}
+
+/// A result of selector matching, includes 3 failure types,
+///
+///   NotMatchedAndRestartFromClosestLaterSibling
+///   NotMatchedAndRestartFromClosestDescendant
+///   NotMatchedGlobally
+///
+/// When NotMatchedGlobally appears, stop selector matching completely since
+/// the succeeding selectors never matches.
+/// It is raised when
+///   Child combinator cannot find the candidate element.
+///   Descendant combinator cannot find the candidate element.
+///
+/// When NotMatchedAndRestartFromClosestDescendant appears, the selector
+/// matching does backtracking and restarts from the closest Descendant
+/// combinator.
+/// It is raised when
+///   NextSibling combinator cannot find the candidate element.
+///   LaterSibling combinator cannot find the candidate element.
+///   Child combinator doesn't match on the found element.
+///
+/// When NotMatchedAndRestartFromClosestLaterSibling appears, the selector
+/// matching does backtracking and restarts from the closest LaterSibling
+/// combinator.
+/// It is raised when
+///   NextSibling combinator doesn't match on the found element.
+///
+/// For example, when the selector "d1 d2 a" is provided and we cannot *find*
+/// an appropriate ancestor element for "d1", this selector matching raises
+/// NotMatchedGlobally since even if "d2" is moved to more upper element, the
+/// candidates for "d1" becomes less than before and d1 .
+///
+/// The next example is siblings. When the selector "b1 + b2 ~ d1 a" is
+/// provided and we cannot *find* an appropriate brother element for b1,
+/// the selector matching raises NotMatchedAndRestartFromClosestDescendant.
+/// The selectors ("b1 + b2 ~") doesn't match and matching restart from "d1".
+///
+/// The additional example is child and sibling. When the selector
+/// "b1 + c1 > b2 ~ d1 a" is provided and the selector "b1" doesn't match on
+/// the element, this "b1" raises NotMatchedAndRestartFromClosestLaterSibling.
+/// However since the selector "c1" raises
+/// NotMatchedAndRestartFromClosestDescendant. So the selector
+/// "b1 + c1 > b2 ~ " doesn't match and restart matching from "d1".
+#[derive(Clone, Copy, Eq, PartialEq)]
+enum SelectorMatchingResult {
+    Matched,
+    NotMatchedAndRestartFromClosestLaterSibling,
+    NotMatchedAndRestartFromClosestDescendant,
+    NotMatchedGlobally,
+}
+
+/// Matches a selector, fast-rejecting against a bloom filter.
+///
+/// We accept an offset to allow consumers to represent and match against
+/// partial selectors (indexed from the right). We use this API design, rather
+/// than having the callers pass a SelectorIter, because creating a SelectorIter
+/// requires dereferencing the selector to get the length, which adds an
+/// unncessary cache miss for cases when we can fast-reject with AncestorHashes
+/// (which the caller can store inline with the selector pointer).
+#[inline(always)]
+pub fn matches_selector<E>(
+    selector: &Selector<E::Impl>,
+    offset: usize,
+    hashes: Option<&AncestorHashes>,
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+) -> bool
+where
+    E: Element,
+{
+    // Use the bloom filter to fast-reject.
+    if let Some(hashes) = hashes {
+        if let Some(filter) = context.bloom_filter {
+            if !may_match(hashes, filter) {
+                return false;
+            }
+        }
+    }
+
+    matches_complex_selector(selector.iter_from(offset), element, context)
+}
+
+/// Whether a compound selector matched, and whether it was the rightmost
+/// selector inside the complex selector.
+pub enum CompoundSelectorMatchingResult {
+    /// The selector was fully matched.
+    FullyMatched,
+    /// The compound selector matched, and the next combinator offset is
+    /// `next_combinator_offset`.
+    Matched { next_combinator_offset: usize },
+    /// The selector didn't match.
+    NotMatched,
+}
+
+/// Matches a compound selector belonging to `selector`, starting at offset
+/// `from_offset`, matching left to right.
+///
+/// Requires that `from_offset` points to a `Combinator`.
+///
+/// NOTE(emilio): This doesn't allow to match in the leftmost sequence of the
+/// complex selector, but it happens to be the case we don't need it.
+pub fn matches_compound_selector_from<E>(
+    selector: &Selector<E::Impl>,
+    mut from_offset: usize,
+    context: &mut MatchingContext<E::Impl>,
+    element: &E,
+) -> CompoundSelectorMatchingResult
+where
+    E: Element,
+{
+    if cfg!(debug_assertions) && from_offset != 0 {
+        selector.combinator_at_parse_order(from_offset - 1); // This asserts.
+    }
+
+    let mut local_context = LocalMatchingContext {
+        shared: context,
+        quirks_data: None,
+    };
+
+    // Find the end of the selector or the next combinator, then match
+    // backwards, so that we match in the same order as
+    // matches_complex_selector, which is usually faster.
+    let start_offset = from_offset;
+    for component in selector.iter_raw_parse_order_from(from_offset) {
+        if matches!(*component, Component::Combinator(..)) {
+            debug_assert_ne!(from_offset, 0, "Selector started with a combinator?");
+            break;
+        }
+
+        from_offset += 1;
+    }
+
+    debug_assert!(from_offset >= 1);
+    debug_assert!(from_offset <= selector.len());
+
+    let iter = selector.iter_from(selector.len() - from_offset);
+    debug_assert!(
+        iter.clone().next().is_some() ||
+            (from_offset != selector.len() &&
+                matches!(
+                    selector.combinator_at_parse_order(from_offset),
+                    Combinator::SlotAssignment | Combinator::PseudoElement
+                )),
+        "Got the math wrong: {:?} | {:?} | {} {}",
+        selector,
+        selector.iter_raw_match_order().as_slice(),
+        from_offset,
+        start_offset
+    );
+
+    for component in iter {
+        if !matches_simple_selector(component, element, &mut local_context) {
+            return CompoundSelectorMatchingResult::NotMatched;
+        }
+    }
+
+    if from_offset != selector.len() {
+        return CompoundSelectorMatchingResult::Matched {
+            next_combinator_offset: from_offset,
+        };
+    }
+
+    CompoundSelectorMatchingResult::FullyMatched
+}
+
+/// Matches a complex selector.
+#[inline(always)]
+pub fn matches_complex_selector<E>(
+    mut iter: SelectorIter<E::Impl>,
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+) -> bool
+where
+    E: Element,
+{
+    // If this is the special pseudo-element mode, consume the ::pseudo-element
+    // before proceeding, since the caller has already handled that part.
+    if context.matching_mode() == MatchingMode::ForStatelessPseudoElement && !context.is_nested() {
+        // Consume the pseudo.
+        match *iter.next().unwrap() {
+            Component::PseudoElement(ref pseudo) => {
+                if let Some(ref f) = context.pseudo_element_matching_fn {
+                    if !f(pseudo) {
+                        return false;
+                    }
+                }
+            },
+            _ => {
+                debug_assert!(
+                    false,
+                    "Used MatchingMode::ForStatelessPseudoElement \
+                     in a non-pseudo selector"
+                );
+            },
+        }
+
+        if !iter.matches_for_stateless_pseudo_element() {
+            return false;
+        }
+
+        // Advance to the non-pseudo-element part of the selector.
+        let next_sequence = iter.next_sequence().unwrap();
+        debug_assert_eq!(next_sequence, Combinator::PseudoElement);
+    }
+
+    let result = matches_complex_selector_internal(iter, element, context, Rightmost::Yes);
+
+    matches!(result, SelectorMatchingResult::Matched)
+}
+
+/// Traverse all descendents of the given element and return true as soon as any of them match
+/// the given list of selectors.
+fn has_children_matching<E: Element>(
+    selectors: &[Selector<E::Impl>],
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+) -> bool {
+    let mut current = element.first_element_child();
+
+    while let Some(el) = current {
+        for selector in selectors {
+            if matches_complex_selector(selector.iter(), &el, context) {
+                return true;
+            }
+        }
+
+        if has_children_matching(selectors, &el, context) {
+            return true;
+        }
+
+        current = el.next_sibling_element();
+    }
+
+    false
+}
+
+/// Whether the :hover and :active quirk applies.
+///
+/// https://quirks.spec.whatwg.org/#the-active-and-hover-quirk
+fn hover_and_active_quirk_applies<Impl: SelectorImpl>(
+    selector_iter: &SelectorIter<Impl>,
+    context: &MatchingContext<Impl>,
+    rightmost: Rightmost,
+) -> bool {
+    if context.quirks_mode() != QuirksMode::Quirks {
+        return false;
+    }
+
+    if context.is_nested() {
+        return false;
+    }
+
+    // This compound selector had a pseudo-element to the right that we
+    // intentionally skipped.
+    if rightmost == Rightmost::Yes &&
+        context.matching_mode() == MatchingMode::ForStatelessPseudoElement
+    {
+        return false;
+    }
+
+    selector_iter.clone().all(|simple| match *simple {
+        Component::LocalName(_) |
+        Component::AttributeInNoNamespaceExists { .. } |
+        Component::AttributeInNoNamespace { .. } |
+        Component::AttributeOther(_) |
+        Component::ID(_) |
+        Component::Class(_) |
+        Component::PseudoElement(_) |
+        Component::Negation(_) |
+        Component::Empty |
+        Component::Nth(_) |
+        Component::NthOf(_) => false,
+        Component::NonTSPseudoClass(ref pseudo_class) => pseudo_class.is_active_or_hover(),
+        _ => true,
+    })
+}
+
+#[derive(Clone, Copy, PartialEq)]
+enum Rightmost {
+    Yes,
+    No,
+}
+
+#[inline(always)]
+fn next_element_for_combinator<E>(
+    element: &E,
+    combinator: Combinator,
+    selector: &SelectorIter<E::Impl>,
+    context: &MatchingContext<E::Impl>,
+) -> Option<E>
+where
+    E: Element,
+{
+    match combinator {
+        Combinator::NextSibling | Combinator::LaterSibling => element.prev_sibling_element(),
+        Combinator::Child | Combinator::Descendant => {
+            match element.parent_element() {
+                Some(e) => return Some(e),
+                None => {},
+            }
+
+            if !element.parent_node_is_shadow_root() {
+                return None;
+            }
+
+            // https://drafts.csswg.org/css-scoping/#host-element-in-tree:
+            //
+            //   For the purpose of Selectors, a shadow host also appears in
+            //   its shadow tree, with the contents of the shadow tree treated
+            //   as its children. (In other words, the shadow host is treated as
+            //   replacing the shadow root node.)
+            //
+            // and also:
+            //
+            //   When considered within its own shadow trees, the shadow host is
+            //   featureless. Only the :host, :host(), and :host-context()
+            //   pseudo-classes are allowed to match it.
+            //
+            // Since we know that the parent is a shadow root, we necessarily
+            // are in a shadow tree of the host, and the next selector will only
+            // match if the selector is a featureless :host selector.
+            if !selector.clone().is_featureless_host_selector() {
+                return None;
+            }
+
+            element.containing_shadow_host()
+        },
+        Combinator::Part => element.containing_shadow_host(),
+        Combinator::SlotAssignment => {
+            debug_assert!(element
+                .assigned_slot()
+                .map_or(true, |s| s.is_html_slot_element()));
+            let scope = context.current_host?;
+            let mut current_slot = element.assigned_slot()?;
+            while current_slot.containing_shadow_host().unwrap().opaque() != scope {
+                current_slot = current_slot.assigned_slot()?;
+            }
+            Some(current_slot)
+        },
+        Combinator::PseudoElement => element.pseudo_element_originating_element(),
+    }
+}
+
+fn matches_complex_selector_internal<E>(
+    mut selector_iter: SelectorIter<E::Impl>,
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+    rightmost: Rightmost,
+) -> SelectorMatchingResult
+where
+    E: Element,
+{
+    debug!(
+        "Matching complex selector {:?} for {:?}",
+        selector_iter, element
+    );
+
+    let matches_compound_selector =
+        matches_compound_selector(&mut selector_iter, element, context, rightmost);
+
+    let combinator = selector_iter.next_sequence();
+    if combinator.map_or(false, |c| c.is_sibling()) {
+        if context.needs_selector_flags() {
+            element.apply_selector_flags(ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS);
+        }
+    }
+
+    if !matches_compound_selector {
+        return SelectorMatchingResult::NotMatchedAndRestartFromClosestLaterSibling;
+    }
+
+    let combinator = match combinator {
+        None => return SelectorMatchingResult::Matched,
+        Some(c) => c,
+    };
+
+    let candidate_not_found = match combinator {
+        Combinator::NextSibling | Combinator::LaterSibling => {
+            SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant
+        },
+        Combinator::Child |
+        Combinator::Descendant |
+        Combinator::SlotAssignment |
+        Combinator::Part |
+        Combinator::PseudoElement => SelectorMatchingResult::NotMatchedGlobally,
+    };
+
+    // Stop matching :visited as soon as we find a link, or a combinator for
+    // something that isn't an ancestor.
+    let mut visited_handling = if combinator.is_sibling() {
+        VisitedHandlingMode::AllLinksUnvisited
+    } else {
+        context.visited_handling()
+    };
+
+    let mut element = element.clone();
+    loop {
+        if element.is_link() {
+            visited_handling = VisitedHandlingMode::AllLinksUnvisited;
+        }
+
+        element = match next_element_for_combinator(&element, combinator, &selector_iter, &context)
+        {
+            None => return candidate_not_found,
+            Some(next_element) => next_element,
+        };
+
+        let result = context.with_visited_handling_mode(visited_handling, |context| {
+            matches_complex_selector_internal(
+                selector_iter.clone(),
+                &element,
+                context,
+                Rightmost::No,
+            )
+        });
+
+        match (result, combinator) {
+            // Return the status immediately.
+            (SelectorMatchingResult::Matched, _) |
+            (SelectorMatchingResult::NotMatchedGlobally, _) |
+            (_, Combinator::NextSibling) => {
+                return result;
+            },
+
+            // Upgrade the failure status to
+            // NotMatchedAndRestartFromClosestDescendant.
+            (_, Combinator::PseudoElement) | (_, Combinator::Child) => {
+                return SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant;
+            },
+
+            // If the failure status is
+            // NotMatchedAndRestartFromClosestDescendant and combinator is
+            // Combinator::LaterSibling, give up this Combinator::LaterSibling
+            // matching and restart from the closest descendant combinator.
+            (
+                SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant,
+                Combinator::LaterSibling,
+            ) => {
+                return result;
+            },
+
+            // The Combinator::Descendant combinator and the status is
+            // NotMatchedAndRestartFromClosestLaterSibling or
+            // NotMatchedAndRestartFromClosestDescendant, or the
+            // Combinator::LaterSibling combinator and the status is
+            // NotMatchedAndRestartFromClosestDescendant, we can continue to
+            // matching on the next candidate element.
+            _ => {},
+        }
+    }
+}
+
+#[inline]
+fn matches_local_name<E>(element: &E, local_name: &LocalName<E::Impl>) -> bool
+where
+    E: Element,
+{
+    let name = select_name(element, &local_name.name, &local_name.lower_name).borrow();
+    element.has_local_name(name)
+}
+
+/// Determines whether the given element matches the given compound selector.
+#[inline]
+fn matches_compound_selector<E>(
+    selector_iter: &mut SelectorIter<E::Impl>,
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+    rightmost: Rightmost,
+) -> bool
+where
+    E: Element,
+{
+    let quirks_data = if context.quirks_mode() == QuirksMode::Quirks {
+        Some((rightmost, selector_iter.clone()))
+    } else {
+        None
+    };
+
+    // Handle some common cases first.
+    // We may want to get rid of this at some point if we can make the
+    // generic case fast enough.
+    let mut selector = selector_iter.next();
+    if let Some(&Component::LocalName(ref local_name)) = selector {
+        if !matches_local_name(element, local_name) {
+            return false;
+        }
+        selector = selector_iter.next();
+    }
+    let class_and_id_case_sensitivity = context.classes_and_ids_case_sensitivity();
+    if let Some(&Component::ID(ref id)) = selector {
+        if !element.has_id(id, class_and_id_case_sensitivity) {
+            return false;
+        }
+        selector = selector_iter.next();
+    }
+    while let Some(&Component::Class(ref class)) = selector {
+        if !element.has_class(class, class_and_id_case_sensitivity) {
+            return false;
+        }
+        selector = selector_iter.next();
+    }
+    let selector = match selector {
+        Some(s) => s,
+        None => return true,
+    };
+
+    let mut local_context = LocalMatchingContext {
+        shared: context,
+        quirks_data,
+    };
+    iter::once(selector)
+        .chain(selector_iter)
+        .all(|simple| matches_simple_selector(simple, element, &mut local_context))
+}
+
+/// Determines whether the given element matches the given single selector.
+fn matches_simple_selector<E>(
+    selector: &Component<E::Impl>,
+    element: &E,
+    context: &mut LocalMatchingContext<E::Impl>,
+) -> bool
+where
+    E: Element,
+{
+    debug_assert!(context.shared.is_nested() || !context.shared.in_negation());
+
+    match *selector {
+        Component::ID(ref id) => {
+            element.has_id(id, context.shared.classes_and_ids_case_sensitivity())
+        },
+        Component::Class(ref class) => {
+            element.has_class(class, context.shared.classes_and_ids_case_sensitivity())
+        },
+        Component::LocalName(ref local_name) => matches_local_name(element, local_name),
+        Component::AttributeInNoNamespaceExists {
+            ref local_name,
+            ref local_name_lower,
+        } => element.attr_matches(
+            &NamespaceConstraint::Specific(&crate::parser::namespace_empty_string::<E::Impl>()),
+            select_name(element, local_name, local_name_lower),
+            &AttrSelectorOperation::Exists,
+        ),
+        Component::AttributeInNoNamespace {
+            ref local_name,
+            ref value,
+            operator,
+            case_sensitivity,
+            never_matches,
+        } => {
+            if never_matches {
+                return false;
+            }
+            element.attr_matches(
+                &NamespaceConstraint::Specific(&crate::parser::namespace_empty_string::<E::Impl>()),
+                local_name,
+                &AttrSelectorOperation::WithValue {
+                    operator,
+                    case_sensitivity: to_unconditional_case_sensitivity(case_sensitivity, element),
+                    expected_value: value,
+                },
+            )
+        },
+        Component::AttributeOther(ref attr_sel) => {
+            if attr_sel.never_matches {
+                return false;
+            }
+            let empty_string;
+            let namespace = match attr_sel.namespace() {
+                Some(ns) => ns,
+                None => {
+                    empty_string = crate::parser::namespace_empty_string::<E::Impl>();
+                    NamespaceConstraint::Specific(&empty_string)
+                },
+            };
+            element.attr_matches(
+                &namespace,
+                select_name(element, &attr_sel.local_name, &attr_sel.local_name_lower),
+                &match attr_sel.operation {
+                    ParsedAttrSelectorOperation::Exists => AttrSelectorOperation::Exists,
+                    ParsedAttrSelectorOperation::WithValue {
+                        operator,
+                        case_sensitivity,
+                        ref expected_value,
+                    } => AttrSelectorOperation::WithValue {
+                        operator,
+                        case_sensitivity: to_unconditional_case_sensitivity(
+                            case_sensitivity,
+                            element,
+                        ),
+                        expected_value,
+                    },
+                },
+            )
+        },
+        Component::Part(ref parts) => {
+            let mut hosts = SmallVec::<[E; 4]>::new();
+
+            let mut host = match element.containing_shadow_host() {
+                Some(h) => h,
+                None => return false,
+            };
+
+            let current_host = context.shared.current_host;
+            if current_host != Some(host.opaque()) {
+                loop {
+                    let outer_host = host.containing_shadow_host();
+                    if outer_host.as_ref().map(|h| h.opaque()) == current_host {
+                        break;
+                    }
+                    let outer_host = match outer_host {
+                        Some(h) => h,
+                        None => return false,
+                    };
+                    // TODO(emilio): if worth it, we could early return if
+                    // host doesn't have the exportparts attribute.
+                    hosts.push(host);
+                    host = outer_host;
+                }
+            }
+
+            // Translate the part into the right scope.
+            parts.iter().all(|part| {
+                let mut part = part.clone();
+                for host in hosts.iter().rev() {
+                    part = match host.imported_part(&part) {
+                        Some(p) => p,
+                        None => return false,
+                    };
+                }
+                element.is_part(&part)
+            })
+        },
+        Component::Slotted(ref selector) => {
+            // <slots> are never flattened tree slottables.
+            !element.is_html_slot_element() &&
+                context
+                    .shared
+                    .nest(|context| matches_complex_selector(selector.iter(), element, context))
+        },
+        Component::PseudoElement(ref pseudo) => {
+            element.match_pseudo_element(pseudo, context.shared)
+        },
+        Component::ExplicitUniversalType | Component::ExplicitAnyNamespace => true,
+        Component::Namespace(_, ref url) | Component::DefaultNamespace(ref url) => {
+            element.has_namespace(&url.borrow())
+        },
+        Component::ExplicitNoNamespace => {
+            let ns = crate::parser::namespace_empty_string::<E::Impl>();
+            element.has_namespace(&ns.borrow())
+        },
+        Component::NonTSPseudoClass(ref pc) => {
+            if let Some((ref rightmost, ref iter)) = context.quirks_data {
+                if pc.is_active_or_hover() &&
+                    !element.is_link() &&
+                    hover_and_active_quirk_applies(iter, context.shared, *rightmost)
+                {
+                    return false;
+                }
+            }
+            element.match_non_ts_pseudo_class(pc, &mut context.shared)
+        },
+        Component::Root => element.is_root(),
+        Component::Empty => {
+            if context.shared.needs_selector_flags() {
+                element.apply_selector_flags(ElementSelectorFlags::HAS_EMPTY_SELECTOR);
+            }
+            element.is_empty()
+        },
+        Component::Host(ref selector) => {
+            context
+                .shared
+                .shadow_host()
+                .map_or(false, |host| host == element.opaque()) &&
+                selector.as_ref().map_or(true, |selector| {
+                    context
+                        .shared
+                        .nest(|context| matches_complex_selector(selector.iter(), element, context))
+                })
+        },
+        Component::Scope => match context.shared.scope_element {
+            Some(ref scope_element) => element.opaque() == *scope_element,
+            None => element.is_root(),
+        },
+        Component::Nth(ref nth_data) => {
+            matches_generic_nth_child(element, context.shared, nth_data)
+        },
+        Component::NthOf(ref nth_of_data) => {
+            // TODO(zrhoffman, bug 1808228): Use selectors() when matching
+            matches_generic_nth_child(element, context.shared, nth_of_data.nth_data())
+        },
+        Component::Is(ref list) | Component::Where(ref list) => context.shared.nest(|context| {
+            for selector in &**list {
+                if matches_complex_selector(selector.iter(), element, context) {
+                    return true;
+                }
+            }
+            false
+        }),
+        Component::Negation(ref list) => context.shared.nest_for_negation(|context| {
+            for selector in &**list {
+                if matches_complex_selector(selector.iter(), element, context) {
+                    return false;
+                }
+            }
+            true
+        }),
+        Component::Has(ref list) => context
+            .shared
+            .nest(|context| has_children_matching(list, element, context)),
+        Component::Combinator(_) => unsafe {
+            debug_unreachable!("Shouldn't try to selector-match combinators")
+        },
+    }
+}
+
+#[inline(always)]
+fn select_name<'a, E: Element, T: PartialEq>(
+    element: &E,
+    local_name: &'a T,
+    local_name_lower: &'a T,
+) -> &'a T {
+    if local_name == local_name_lower || element.is_html_element_in_html_document() {
+        local_name_lower
+    } else {
+        local_name
+    }
+}
+
+#[inline(always)]
+fn to_unconditional_case_sensitivity<'a, E: Element>(
+    parsed: ParsedCaseSensitivity,
+    element: &E,
+) -> CaseSensitivity {
+    match parsed {
+        ParsedCaseSensitivity::CaseSensitive | ParsedCaseSensitivity::ExplicitCaseSensitive => {
+            CaseSensitivity::CaseSensitive
+        },
+        ParsedCaseSensitivity::AsciiCaseInsensitive => CaseSensitivity::AsciiCaseInsensitive,
+        ParsedCaseSensitivity::AsciiCaseInsensitiveIfInHtmlElementInHtmlDocument => {
+            if element.is_html_element_in_html_document() {
+                CaseSensitivity::AsciiCaseInsensitive
+            } else {
+                CaseSensitivity::CaseSensitive
+            }
+        },
+    }
+}
+
+fn matches_generic_nth_child<E>(
+    element: &E,
+    context: &mut MatchingContext<E::Impl>,
+    nth_data: &NthSelectorData,
+) -> bool
+where
+    E: Element,
+{
+    if element.ignores_nth_child_selectors() {
+        return false;
+    }
+
+    let NthSelectorData { ty, a, b, .. } = *nth_data;
+    let is_of_type = ty.is_of_type();
+    if ty.is_only() {
+        return matches_generic_nth_child(element, context, &NthSelectorData::first(is_of_type)) &&
+            matches_generic_nth_child(element, context, &NthSelectorData::last(is_of_type));
+    }
+
+    let is_from_end = ty.is_from_end();
+
+    // It's useful to know whether this can only select the first/last element
+    // child for optimization purposes, see the `HAS_EDGE_CHILD_SELECTOR` flag.
+    let is_edge_child_selector = a == 0 && b == 1 && !is_of_type;
+
+    if context.needs_selector_flags() {
+        element.apply_selector_flags(if is_edge_child_selector {
+            ElementSelectorFlags::HAS_EDGE_CHILD_SELECTOR
+        } else if is_from_end {
+            ElementSelectorFlags::HAS_SLOW_SELECTOR
+        } else {
+            ElementSelectorFlags::HAS_SLOW_SELECTOR_LATER_SIBLINGS
+        });
+    }
+
+    // :first/last-child are rather trivial to match, don't bother with the
+    // cache.
+    if is_edge_child_selector {
+        return if is_from_end {
+            element.next_sibling_element()
+        } else {
+            element.prev_sibling_element()
+        }
+        .is_none();
+    }
+
+    // Grab a reference to the appropriate cache.
+    let mut cache = context
+        .nth_index_cache
+        .as_mut()
+        .map(|c| c.get(is_of_type, is_from_end));
+
+    // Lookup or compute the index.
+    let index = if let Some(i) = cache.as_mut().and_then(|c| c.lookup(element.opaque())) {
+        i
+    } else {
+        let i = nth_child_index(element, is_of_type, is_from_end, cache.as_deref_mut());
+        if let Some(c) = cache.as_mut() {
+            c.insert(element.opaque(), i)
+        }
+        i
+    };
+    debug_assert_eq!(
+        index,
+        nth_child_index(element, is_of_type, is_from_end, None),
+        "invalid cache"
+    );
+
+    // Is there a non-negative integer n such that An+B=index?
+    match index.checked_sub(b) {
+        None => false,
+        Some(an) => match an.checked_div(a) {
+            Some(n) => n >= 0 && a * n == an,
+            None /* a == 0 */ => an == 0,
+        },
+    }
+}
+
+#[inline]
+fn nth_child_index<E>(
+    element: &E,
+    is_of_type: bool,
+    is_from_end: bool,
+    mut cache: Option<&mut NthIndexCacheInner>,
+) -> i32
+where
+    E: Element,
+{
+    // The traversal mostly processes siblings left to right. So when we walk
+    // siblings to the right when computing NthLast/NthLastOfType we're unlikely
+    // to get cache hits along the way. As such, we take the hit of walking the
+    // siblings to the left checking the cache in the is_from_end case (this
+    // matches what Gecko does). The indices-from-the-left is handled during the
+    // regular look further below.
+    if let Some(ref mut c) = cache {
+        if is_from_end && !c.is_empty() {
+            let mut index: i32 = 1;
+            let mut curr = element.clone();
+            while let Some(e) = curr.prev_sibling_element() {
+                curr = e;
+                if !is_of_type || element.is_same_type(&curr) {
+                    if let Some(i) = c.lookup(curr.opaque()) {
+                        return i - index;
+                    }
+                    index += 1;
+                }
+            }
+        }
+    }
+
+    let mut index: i32 = 1;
+    let mut curr = element.clone();
+    let next = |e: E| {
+        if is_from_end {
+            e.next_sibling_element()
+        } else {
+            e.prev_sibling_element()
+        }
+    };
+    while let Some(e) = next(curr) {
+        curr = e;
+        if !is_of_type || element.is_same_type(&curr) {
+            // If we're computing indices from the left, check each element in the
+            // cache. We handle the indices-from-the-right case at the top of this
+            // function.
+            if !is_from_end {
+                if let Some(i) = cache.as_mut().and_then(|c| c.lookup(curr.opaque())) {
+                    return i + index;
+                }
+            }
+            index += 1;
+        }
+    }
+
+    index
+}
diff --git a/servo/components/selectors/nth_index_cache.rs b/servo/components/selectors/nth_index_cache.rs
new file mode 100644
index 0000000000..979c310017
--- /dev/null
+++ b/servo/components/selectors/nth_index_cache.rs
@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::tree::OpaqueElement;
+use fxhash::FxHashMap;
+
+/// A cache to speed up matching of nth-index-like selectors.
+///
+/// See [1] for some discussion around the design tradeoffs.
+///
+/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1401855#c3
+#[derive(Default)]
+pub struct NthIndexCache {
+    nth: NthIndexCacheInner,
+    nth_last: NthIndexCacheInner,
+    nth_of_type: NthIndexCacheInner,
+    nth_last_of_type: NthIndexCacheInner,
+}
+
+impl NthIndexCache {
+    /// Gets the appropriate cache for the given parameters.
+    pub fn get(&mut self, is_of_type: bool, is_from_end: bool) -> &mut NthIndexCacheInner {
+        match (is_of_type, is_from_end) {
+            (false, false) => &mut self.nth,
+            (false, true) => &mut self.nth_last,
+            (true, false) => &mut self.nth_of_type,
+            (true, true) => &mut self.nth_last_of_type,
+        }
+    }
+}
+
+/// The concrete per-pseudo-class cache.
+#[derive(Default)]
+pub struct NthIndexCacheInner(FxHashMap<OpaqueElement, i32>);
+
+impl NthIndexCacheInner {
+    /// Does a lookup for a given element in the cache.
+    pub fn lookup(&mut self, el: OpaqueElement) -> Option<i32> {
+        self.0.get(&el).copied()
+    }
+
+    /// Inserts an entry into the cache.
+    pub fn insert(&mut self, element: OpaqueElement, index: i32) {
+        self.0.insert(element, index);
+    }
+
+    /// Returns whether the cache is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
diff --git a/servo/components/selectors/parser.rs b/servo/components/selectors/parser.rs
new file mode 100644
index 0000000000..b5cbd64c25
--- /dev/null
+++ b/servo/components/selectors/parser.rs
@@ -0,0 +1,3547 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::attr::{AttrSelectorOperator, AttrSelectorWithOptionalNamespace};
+use crate::attr::{NamespaceConstraint, ParsedAttrSelectorOperation};
+use crate::attr::{ParsedCaseSensitivity, SELECTOR_WHITESPACE};
+use crate::bloom::BLOOM_HASH_MASK;
+use crate::builder::{SelectorBuilder, SelectorFlags, SpecificityAndFlags};
+use crate::context::QuirksMode;
+use crate::sink::Push;
+pub use crate::visitor::SelectorVisitor;
+use cssparser::parse_nth;
+use cssparser::{BasicParseError, BasicParseErrorKind, ParseError, ParseErrorKind};
+use cssparser::{CowRcStr, Delimiter, SourceLocation};
+use cssparser::{Parser as CssParser, ToCss, Token};
+use precomputed_hash::PrecomputedHash;
+use servo_arc::ThinArc;
+use smallvec::SmallVec;
+use std::borrow::{Borrow, Cow};
+use std::fmt::{self, Debug};
+use std::iter::Rev;
+use std::slice;
+
+/// A trait that represents a pseudo-element.
+pub trait PseudoElement: Sized + ToCss {
+    /// The `SelectorImpl` this pseudo-element is used for.
+    type Impl: SelectorImpl;
+
+    /// Whether the pseudo-element supports a given state selector to the right
+    /// of it.
+    fn accepts_state_pseudo_classes(&self) -> bool {
+        false
+    }
+
+    /// Whether this pseudo-element is valid after a ::slotted(..) pseudo.
+    fn valid_after_slotted(&self) -> bool {
+        false
+    }
+}
+
+/// A trait that represents a pseudo-class.
+pub trait NonTSPseudoClass: Sized + ToCss {
+    /// The `SelectorImpl` this pseudo-element is used for.
+    type Impl: SelectorImpl;
+
+    /// Whether this pseudo-class is :active or :hover.
+    fn is_active_or_hover(&self) -> bool;
+
+    /// Whether this pseudo-class belongs to:
+    ///
+    /// https://drafts.csswg.org/selectors-4/#useraction-pseudos
+    fn is_user_action_state(&self) -> bool;
+
+    fn visit<V>(&self, _visitor: &mut V) -> bool
+    where
+        V: SelectorVisitor<Impl = Self::Impl>,
+    {
+        true
+    }
+}
+
+/// Returns a Cow::Borrowed if `s` is already ASCII lowercase, and a
+/// Cow::Owned if `s` had to be converted into ASCII lowercase.
+fn to_ascii_lowercase(s: &str) -> Cow<str> {
+    if let Some(first_uppercase) = s.bytes().position(|byte| byte >= b'A' && byte <= b'Z') {
+        let mut string = s.to_owned();
+        string[first_uppercase..].make_ascii_lowercase();
+        string.into()
+    } else {
+        s.into()
+    }
+}
+
+bitflags! {
+    /// Flags that indicate at which point of parsing a selector are we.
+    struct SelectorParsingState: u8 {
+        /// Whether we should avoid adding default namespaces to selectors that
+        /// aren't type or universal selectors.
+        const SKIP_DEFAULT_NAMESPACE = 1 << 0;
+
+        /// Whether we've parsed a ::slotted() pseudo-element already.
+        ///
+        /// If so, then we can only parse a subset of pseudo-elements, and
+        /// whatever comes after them if so.
+        const AFTER_SLOTTED = 1 << 1;
+        /// Whether we've parsed a ::part() pseudo-element already.
+        ///
+        /// If so, then we can only parse a subset of pseudo-elements, and
+        /// whatever comes after them if so.
+        const AFTER_PART = 1 << 2;
+        /// Whether we've parsed a pseudo-element (as in, an
+        /// `Impl::PseudoElement` thus not accounting for `::slotted` or
+        /// `::part`) already.
+        ///
+        /// If so, then other pseudo-elements and most other selectors are
+        /// disallowed.
+        const AFTER_PSEUDO_ELEMENT = 1 << 3;
+        /// Whether we've parsed a non-stateful pseudo-element (again, as-in
+        /// `Impl::PseudoElement`) already. If so, then other pseudo-classes are
+        /// disallowed. If this flag is set, `AFTER_PSEUDO_ELEMENT` must be set
+        /// as well.
+        const AFTER_NON_STATEFUL_PSEUDO_ELEMENT = 1 << 4;
+
+        /// Whether we are after any of the pseudo-like things.
+        const AFTER_PSEUDO = Self::AFTER_PART.bits | Self::AFTER_SLOTTED.bits | Self::AFTER_PSEUDO_ELEMENT.bits;
+
+        /// Whether we explicitly disallow combinators.
+        const DISALLOW_COMBINATORS = 1 << 5;
+
+        /// Whether we explicitly disallow pseudo-element-like things.
+        const DISALLOW_PSEUDOS = 1 << 6;
+    }
+}
+
+impl SelectorParsingState {
+    #[inline]
+    fn allows_pseudos(self) -> bool {
+        // NOTE(emilio): We allow pseudos after ::part and such.
+        !self.intersects(Self::AFTER_PSEUDO_ELEMENT | Self::DISALLOW_PSEUDOS)
+    }
+
+    #[inline]
+    fn allows_slotted(self) -> bool {
+        !self.intersects(Self::AFTER_PSEUDO | Self::DISALLOW_PSEUDOS)
+    }
+
+    #[inline]
+    fn allows_part(self) -> bool {
+        !self.intersects(Self::AFTER_PSEUDO | Self::DISALLOW_PSEUDOS)
+    }
+
+    // TODO(emilio): Maybe some of these should be allowed, but this gets us on
+    // the safe side for now, matching previous behavior. Gotta be careful with
+    // the ones like :-moz-any, which allow nested selectors but don't carry the
+    // state, and so on.
+    #[inline]
+    fn allows_custom_functional_pseudo_classes(self) -> bool {
+        !self.intersects(Self::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_non_functional_pseudo_classes(self) -> bool {
+        !self.intersects(Self::AFTER_SLOTTED | Self::AFTER_NON_STATEFUL_PSEUDO_ELEMENT)
+    }
+
+    #[inline]
+    fn allows_tree_structural_pseudo_classes(self) -> bool {
+        !self.intersects(Self::AFTER_PSEUDO)
+    }
+
+    #[inline]
+    fn allows_combinators(self) -> bool {
+        !self.intersects(Self::DISALLOW_COMBINATORS)
+    }
+}
+
+pub type SelectorParseError<'i> = ParseError<'i, SelectorParseErrorKind<'i>>;
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum SelectorParseErrorKind<'i> {
+    NoQualifiedNameInAttributeSelector(Token<'i>),
+    EmptySelector,
+    DanglingCombinator,
+    NonCompoundSelector,
+    NonPseudoElementAfterSlotted,
+    InvalidPseudoElementAfterSlotted,
+    InvalidPseudoElementInsideWhere,
+    InvalidState,
+    UnexpectedTokenInAttributeSelector(Token<'i>),
+    PseudoElementExpectedColon(Token<'i>),
+    PseudoElementExpectedIdent(Token<'i>),
+    NoIdentForPseudo(Token<'i>),
+    UnsupportedPseudoClassOrElement(CowRcStr<'i>),
+    UnexpectedIdent(CowRcStr<'i>),
+    ExpectedNamespace(CowRcStr<'i>),
+    ExpectedBarInAttr(Token<'i>),
+    BadValueInAttr(Token<'i>),
+    InvalidQualNameInAttr(Token<'i>),
+    ExplicitNamespaceUnexpectedToken(Token<'i>),
+    ClassNeedsIdent(Token<'i>),
+}
+
+macro_rules! with_all_bounds {
+    (
+        [ $( $InSelector: tt )* ]
+        [ $( $CommonBounds: tt )* ]
+        [ $( $FromStr: tt )* ]
+    ) => {
+        /// This trait allows to define the parser implementation in regards
+        /// of pseudo-classes/elements
+        ///
+        /// NB: We need Clone so that we can derive(Clone) on struct with that
+        /// are parameterized on SelectorImpl. See
+        /// <https://github.com/rust-lang/rust/issues/26925>
+        pub trait SelectorImpl: Clone + Debug + Sized + 'static {
+            type ExtraMatchingData<'a>: Sized + Default;
+            type AttrValue: $($InSelector)*;
+            type Identifier: $($InSelector)*;
+            type LocalName: $($InSelector)* + Borrow<Self::BorrowedLocalName>;
+            type NamespaceUrl: $($CommonBounds)* + Default + Borrow<Self::BorrowedNamespaceUrl>;
+            type NamespacePrefix: $($InSelector)* + Default;
+            type BorrowedNamespaceUrl: ?Sized + Eq;
+            type BorrowedLocalName: ?Sized + Eq;
+
+            /// non tree-structural pseudo-classes
+            /// (see: https://drafts.csswg.org/selectors/#structural-pseudos)
+            type NonTSPseudoClass: $($CommonBounds)* + NonTSPseudoClass<Impl = Self>;
+
+            /// pseudo-elements
+            type PseudoElement: $($CommonBounds)* + PseudoElement<Impl = Self>;
+
+            /// Whether attribute hashes should be collected for filtering
+            /// purposes.
+            fn should_collect_attr_hash(_name: &Self::LocalName) -> bool {
+                false
+            }
+        }
+    }
+}
+
+macro_rules! with_bounds {
+    ( [ $( $CommonBounds: tt )* ] [ $( $FromStr: tt )* ]) => {
+        with_all_bounds! {
+            [$($CommonBounds)* + $($FromStr)* + ToCss]
+            [$($CommonBounds)*]
+            [$($FromStr)*]
+        }
+    }
+}
+
+with_bounds! {
+    [Clone + Eq]
+    [for<'a> From<&'a str>]
+}
+
+pub trait Parser<'i> {
+    type Impl: SelectorImpl;
+    type Error: 'i + From<SelectorParseErrorKind<'i>>;
+
+    /// Whether to parse the `::slotted()` pseudo-element.
+    fn parse_slotted(&self) -> bool {
+        false
+    }
+
+    /// Whether to parse the `::part()` pseudo-element.
+    fn parse_part(&self) -> bool {
+        false
+    }
+
+    /// Whether to parse the selector list of nth-child() or nth-last-child().
+    fn parse_nth_child_of(&self) -> bool {
+        false
+    }
+
+    /// Whether to parse the `:where` pseudo-class.
+    fn parse_is_and_where(&self) -> bool {
+        false
+    }
+
+    /// Whether to parse the :has pseudo-class.
+    fn parse_has(&self) -> bool {
+        false
+    }
+
+    /// Whether the given function name is an alias for the `:is()` function.
+    fn is_is_alias(&self, _name: &str) -> bool {
+        false
+    }
+
+    /// Whether to parse the `:host` pseudo-class.
+    fn parse_host(&self) -> bool {
+        false
+    }
+
+    /// Whether to allow forgiving selector-list parsing.
+    fn allow_forgiving_selectors(&self) -> bool {
+        true
+    }
+
+    /// This function can return an "Err" pseudo-element in order to support CSS2.1
+    /// pseudo-elements.
+    fn parse_non_ts_pseudo_class(
+        &self,
+        location: SourceLocation,
+        name: CowRcStr<'i>,
+    ) -> Result<<Self::Impl as SelectorImpl>::NonTSPseudoClass, ParseError<'i, Self::Error>> {
+        Err(
+            location.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                name,
+            )),
+        )
+    }
+
+    fn parse_non_ts_functional_pseudo_class<'t>(
+        &self,
+        name: CowRcStr<'i>,
+        arguments: &mut CssParser<'i, 't>,
+    ) -> Result<<Self::Impl as SelectorImpl>::NonTSPseudoClass, ParseError<'i, Self::Error>> {
+        Err(
+            arguments.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                name,
+            )),
+        )
+    }
+
+    fn parse_pseudo_element(
+        &self,
+        location: SourceLocation,
+        name: CowRcStr<'i>,
+    ) -> Result<<Self::Impl as SelectorImpl>::PseudoElement, ParseError<'i, Self::Error>> {
+        Err(
+            location.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                name,
+            )),
+        )
+    }
+
+    fn parse_functional_pseudo_element<'t>(
+        &self,
+        name: CowRcStr<'i>,
+        arguments: &mut CssParser<'i, 't>,
+    ) -> Result<<Self::Impl as SelectorImpl>::PseudoElement, ParseError<'i, Self::Error>> {
+        Err(
+            arguments.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                name,
+            )),
+        )
+    }
+
+    fn default_namespace(&self) -> Option<<Self::Impl as SelectorImpl>::NamespaceUrl> {
+        None
+    }
+
+    fn namespace_for_prefix(
+        &self,
+        _prefix: &<Self::Impl as SelectorImpl>::NamespacePrefix,
+    ) -> Option<<Self::Impl as SelectorImpl>::NamespaceUrl> {
+        None
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct SelectorList<Impl: SelectorImpl>(
+    #[shmem(field_bound)] pub SmallVec<[Selector<Impl>; 1]>,
+);
+
+/// How to treat invalid selectors in a selector list.
+enum ParseErrorRecovery {
+    /// Discard the entire selector list, this is the default behavior for
+    /// almost all of CSS.
+    DiscardList,
+    /// Ignore invalid selectors, potentially creating an empty selector list.
+    ///
+    /// This is the error recovery mode of :is() and :where()
+    IgnoreInvalidSelector,
+}
+
+impl<Impl: SelectorImpl> SelectorList<Impl> {
+    /// Parse a comma-separated list of Selectors.
+    /// <https://drafts.csswg.org/selectors/#grouping>
+    ///
+    /// Return the Selectors or Err if there is an invalid selector.
+    pub fn parse<'i, 't, P>(
+        parser: &P,
+        input: &mut CssParser<'i, 't>,
+    ) -> Result<Self, ParseError<'i, P::Error>>
+    where
+        P: Parser<'i, Impl = Impl>,
+    {
+        Self::parse_with_state(
+            parser,
+            input,
+            SelectorParsingState::empty(),
+            ParseErrorRecovery::DiscardList,
+        )
+    }
+
+    #[inline]
+    fn parse_with_state<'i, 't, P>(
+        parser: &P,
+        input: &mut CssParser<'i, 't>,
+        state: SelectorParsingState,
+        recovery: ParseErrorRecovery,
+    ) -> Result<Self, ParseError<'i, P::Error>>
+    where
+        P: Parser<'i, Impl = Impl>,
+    {
+        let mut values = SmallVec::new();
+        loop {
+            let selector = input.parse_until_before(Delimiter::Comma, |input| {
+                parse_selector(parser, input, state)
+            });
+
+            let was_ok = selector.is_ok();
+            match selector {
+                Ok(selector) => values.push(selector),
+                Err(err) => match recovery {
+                    ParseErrorRecovery::DiscardList => return Err(err),
+                    ParseErrorRecovery::IgnoreInvalidSelector => {
+                        if !parser.allow_forgiving_selectors() {
+                            return Err(err);
+                        }
+                    },
+                },
+            }
+
+            loop {
+                match input.next() {
+                    Err(_) => return Ok(SelectorList(values)),
+                    Ok(&Token::Comma) => break,
+                    Ok(_) => {
+                        debug_assert!(!was_ok, "Shouldn't have got a selector if getting here");
+                    },
+                }
+            }
+        }
+    }
+
+    /// Creates a SelectorList from a Vec of selectors. Used in tests.
+    pub fn from_vec(v: Vec<Selector<Impl>>) -> Self {
+        SelectorList(SmallVec::from_vec(v))
+    }
+}
+
+/// Parses one compound selector suitable for nested stuff like :-moz-any, etc.
+fn parse_inner_compound_selector<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+) -> Result<Selector<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    parse_selector(
+        parser,
+        input,
+        state | SelectorParsingState::DISALLOW_PSEUDOS | SelectorParsingState::DISALLOW_COMBINATORS,
+    )
+}
+
+/// Ancestor hashes for the bloom filter. We precompute these and store them
+/// inline with selectors to optimize cache performance during matching.
+/// This matters a lot.
+///
+/// We use 4 hashes, which is copied from Gecko, who copied it from WebKit.
+/// Note that increasing the number of hashes here will adversely affect the
+/// cache hit when fast-rejecting long lists of Rules with inline hashes.
+///
+/// Because the bloom filter only uses the bottom 24 bits of the hash, we pack
+/// the fourth hash into the upper bits of the first three hashes in order to
+/// shrink Rule (whose size matters a lot). This scheme minimizes the runtime
+/// overhead of the packing for the first three hashes (we just need to mask
+/// off the upper bits) at the expense of making the fourth somewhat more
+/// complicated to assemble, because we often bail out before checking all the
+/// hashes.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct AncestorHashes {
+    pub packed_hashes: [u32; 3],
+}
+
+fn collect_ancestor_hashes<Impl: SelectorImpl>(
+    iter: SelectorIter<Impl>,
+    quirks_mode: QuirksMode,
+    hashes: &mut [u32; 4],
+    len: &mut usize,
+) -> bool
+where
+    Impl::Identifier: PrecomputedHash,
+    Impl::LocalName: PrecomputedHash,
+    Impl::NamespaceUrl: PrecomputedHash,
+{
+    for component in AncestorIter::new(iter) {
+        let hash = match *component {
+            Component::LocalName(LocalName {
+                ref name,
+                ref lower_name,
+            }) => {
+                // Only insert the local-name into the filter if it's all
+                // lowercase.  Otherwise we would need to test both hashes, and
+                // our data structures aren't really set up for that.
+                if name != lower_name {
+                    continue;
+                }
+                name.precomputed_hash()
+            },
+            Component::DefaultNamespace(ref url) | Component::Namespace(_, ref url) => {
+                url.precomputed_hash()
+            },
+            // In quirks mode, class and id selectors should match
+            // case-insensitively, so just avoid inserting them into the filter.
+            Component::ID(ref id) if quirks_mode != QuirksMode::Quirks => id.precomputed_hash(),
+            Component::Class(ref class) if quirks_mode != QuirksMode::Quirks => {
+                class.precomputed_hash()
+            },
+            Component::AttributeInNoNamespace { ref local_name, .. }
+                if Impl::should_collect_attr_hash(local_name) =>
+            {
+                // AttributeInNoNamespace is only used when local_name ==
+                // local_name_lower.
+                local_name.precomputed_hash()
+            },
+            Component::AttributeInNoNamespaceExists {
+                ref local_name,
+                ref local_name_lower,
+                ..
+            } => {
+                // Only insert the local-name into the filter if it's all
+                // lowercase.  Otherwise we would need to test both hashes, and
+                // our data structures aren't really set up for that.
+                if local_name != local_name_lower || !Impl::should_collect_attr_hash(local_name) {
+                    continue;
+                }
+                local_name.precomputed_hash()
+            },
+            Component::AttributeOther(ref selector) => {
+                if selector.local_name != selector.local_name_lower ||
+                    !Impl::should_collect_attr_hash(&selector.local_name)
+                {
+                    continue;
+                }
+                selector.local_name.precomputed_hash()
+            },
+            Component::Is(ref list) | Component::Where(ref list) => {
+                // :where and :is OR their selectors, so we can't put any hash
+                // in the filter if there's more than one selector, as that'd
+                // exclude elements that may match one of the other selectors.
+                if list.len() == 1 &&
+                    !collect_ancestor_hashes(list[0].iter(), quirks_mode, hashes, len)
+                {
+                    return false;
+                }
+                continue;
+            },
+            _ => continue,
+        };
+
+        hashes[*len] = hash & BLOOM_HASH_MASK;
+        *len += 1;
+        if *len == hashes.len() {
+            return false;
+        }
+    }
+    true
+}
+
+impl AncestorHashes {
+    pub fn new<Impl: SelectorImpl>(selector: &Selector<Impl>, quirks_mode: QuirksMode) -> Self
+    where
+        Impl::Identifier: PrecomputedHash,
+        Impl::LocalName: PrecomputedHash,
+        Impl::NamespaceUrl: PrecomputedHash,
+    {
+        // Compute ancestor hashes for the bloom filter.
+        let mut hashes = [0u32; 4];
+        let mut len = 0;
+        collect_ancestor_hashes(selector.iter(), quirks_mode, &mut hashes, &mut len);
+        debug_assert!(len <= 4);
+
+        // Now, pack the fourth hash (if it exists) into the upper byte of each of
+        // the other three hashes.
+        if len == 4 {
+            let fourth = hashes[3];
+            hashes[0] |= (fourth & 0x000000ff) << 24;
+            hashes[1] |= (fourth & 0x0000ff00) << 16;
+            hashes[2] |= (fourth & 0x00ff0000) << 8;
+        }
+
+        AncestorHashes {
+            packed_hashes: [hashes[0], hashes[1], hashes[2]],
+        }
+    }
+
+    /// Returns the fourth hash, reassembled from parts.
+    pub fn fourth_hash(&self) -> u32 {
+        ((self.packed_hashes[0] & 0xff000000) >> 24) |
+            ((self.packed_hashes[1] & 0xff000000) >> 16) |
+            ((self.packed_hashes[2] & 0xff000000) >> 8)
+    }
+}
+
+pub fn namespace_empty_string<Impl: SelectorImpl>() -> Impl::NamespaceUrl {
+    // Rust type’s default, not default namespace
+    Impl::NamespaceUrl::default()
+}
+
+/// A Selector stores a sequence of simple selectors and combinators. The
+/// iterator classes allow callers to iterate at either the raw sequence level or
+/// at the level of sequences of simple selectors separated by combinators. Most
+/// callers want the higher-level iterator.
+///
+/// We store compound selectors internally right-to-left (in matching order).
+/// Additionally, we invert the order of top-level compound selectors so that
+/// each one matches left-to-right. This is because matching namespace, local name,
+/// id, and class are all relatively cheap, whereas matching pseudo-classes might
+/// be expensive (depending on the pseudo-class). Since authors tend to put the
+/// pseudo-classes on the right, it's faster to start matching on the left.
+///
+/// This reordering doesn't change the semantics of selector matching, and we
+/// handle it in to_css to make it invisible to serialization.
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct Selector<Impl: SelectorImpl>(
+    #[shmem(field_bound)] ThinArc<SpecificityAndFlags, Component<Impl>>,
+);
+
+impl<Impl: SelectorImpl> Selector<Impl> {
+    #[inline]
+    pub fn specificity(&self) -> u32 {
+        self.0.header.header.specificity()
+    }
+
+    #[inline]
+    pub fn has_pseudo_element(&self) -> bool {
+        self.0.header.header.has_pseudo_element()
+    }
+
+    #[inline]
+    pub fn is_slotted(&self) -> bool {
+        self.0.header.header.is_slotted()
+    }
+
+    #[inline]
+    pub fn is_part(&self) -> bool {
+        self.0.header.header.is_part()
+    }
+
+    #[inline]
+    pub fn parts(&self) -> Option<&[Impl::Identifier]> {
+        if !self.is_part() {
+            return None;
+        }
+
+        let mut iter = self.iter();
+        if self.has_pseudo_element() {
+            // Skip the pseudo-element.
+            for _ in &mut iter {}
+
+            let combinator = iter.next_sequence()?;
+            debug_assert_eq!(combinator, Combinator::PseudoElement);
+        }
+
+        for component in iter {
+            if let Component::Part(ref part) = *component {
+                return Some(part);
+            }
+        }
+
+        debug_assert!(false, "is_part() lied somehow?");
+        None
+    }
+
+    #[inline]
+    pub fn pseudo_element(&self) -> Option<&Impl::PseudoElement> {
+        if !self.has_pseudo_element() {
+            return None;
+        }
+
+        for component in self.iter() {
+            if let Component::PseudoElement(ref pseudo) = *component {
+                return Some(pseudo);
+            }
+        }
+
+        debug_assert!(false, "has_pseudo_element lied!");
+        None
+    }
+
+    /// Whether this selector (pseudo-element part excluded) matches every element.
+    ///
+    /// Used for "pre-computed" pseudo-elements in components/style/stylist.rs
+    #[inline]
+    pub fn is_universal(&self) -> bool {
+        self.iter_raw_match_order().all(|c| {
+            matches!(
+                *c,
+                Component::ExplicitUniversalType |
+                    Component::ExplicitAnyNamespace |
+                    Component::Combinator(Combinator::PseudoElement) |
+                    Component::PseudoElement(..)
+            )
+        })
+    }
+
+    /// Returns an iterator over this selector in matching order (right-to-left).
+    /// When a combinator is reached, the iterator will return None, and
+    /// next_sequence() may be called to continue to the next sequence.
+    #[inline]
+    pub fn iter(&self) -> SelectorIter<Impl> {
+        SelectorIter {
+            iter: self.iter_raw_match_order(),
+            next_combinator: None,
+        }
+    }
+
+    /// Whether this selector is a featureless :host selector, with no
+    /// combinators to the left, and optionally has a pseudo-element to the
+    /// right.
+    #[inline]
+    pub fn is_featureless_host_selector_or_pseudo_element(&self) -> bool {
+        let mut iter = self.iter();
+        if !self.has_pseudo_element() {
+            return iter.is_featureless_host_selector();
+        }
+
+        // Skip the pseudo-element.
+        for _ in &mut iter {}
+
+        match iter.next_sequence() {
+            None => return false,
+            Some(combinator) => {
+                debug_assert_eq!(combinator, Combinator::PseudoElement);
+            },
+        }
+
+        iter.is_featureless_host_selector()
+    }
+
+    /// Returns an iterator over this selector in matching order (right-to-left),
+    /// skipping the rightmost |offset| Components.
+    #[inline]
+    pub fn iter_from(&self, offset: usize) -> SelectorIter<Impl> {
+        let iter = self.0.slice[offset..].iter();
+        SelectorIter {
+            iter,
+            next_combinator: None,
+        }
+    }
+
+    /// Returns the combinator at index `index` (zero-indexed from the right),
+    /// or panics if the component is not a combinator.
+    #[inline]
+    pub fn combinator_at_match_order(&self, index: usize) -> Combinator {
+        match self.0.slice[index] {
+            Component::Combinator(c) => c,
+            ref other => panic!(
+                "Not a combinator: {:?}, {:?}, index: {}",
+                other, self, index
+            ),
+        }
+    }
+
+    /// Returns an iterator over the entire sequence of simple selectors and
+    /// combinators, in matching order (from right to left).
+    #[inline]
+    pub fn iter_raw_match_order(&self) -> slice::Iter<Component<Impl>> {
+        self.0.slice.iter()
+    }
+
+    /// Returns the combinator at index `index` (zero-indexed from the left),
+    /// or panics if the component is not a combinator.
+    #[inline]
+    pub fn combinator_at_parse_order(&self, index: usize) -> Combinator {
+        match self.0.slice[self.len() - index - 1] {
+            Component::Combinator(c) => c,
+            ref other => panic!(
+                "Not a combinator: {:?}, {:?}, index: {}",
+                other, self, index
+            ),
+        }
+    }
+
+    /// Returns an iterator over the sequence of simple selectors and
+    /// combinators, in parse order (from left to right), starting from
+    /// `offset`.
+    #[inline]
+    pub fn iter_raw_parse_order_from(&self, offset: usize) -> Rev<slice::Iter<Component<Impl>>> {
+        self.0.slice[..self.len() - offset].iter().rev()
+    }
+
+    /// Creates a Selector from a vec of Components, specified in parse order. Used in tests.
+    #[allow(unused)]
+    pub(crate) fn from_vec(
+        vec: Vec<Component<Impl>>,
+        specificity: u32,
+        flags: SelectorFlags,
+    ) -> Self {
+        let mut builder = SelectorBuilder::default();
+        for component in vec.into_iter() {
+            if let Some(combinator) = component.as_combinator() {
+                builder.push_combinator(combinator);
+            } else {
+                builder.push_simple_selector(component);
+            }
+        }
+        let spec = SpecificityAndFlags { specificity, flags };
+        Selector(builder.build_with_specificity_and_flags(spec))
+    }
+
+    /// Returns count of simple selectors and combinators in the Selector.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.0.slice.len()
+    }
+
+    /// Returns the address on the heap of the ThinArc for memory reporting.
+    pub fn thin_arc_heap_ptr(&self) -> *const ::std::os::raw::c_void {
+        self.0.heap_ptr()
+    }
+
+    /// Traverse selector components inside `self`.
+    ///
+    /// Implementations of this method should call `SelectorVisitor` methods
+    /// or other impls of `Visit` as appropriate based on the fields of `Self`.
+    ///
+    /// A return value of `false` indicates terminating the traversal.
+    /// It should be propagated with an early return.
+    /// On the contrary, `true` indicates that all fields of `self` have been traversed:
+    ///
+    /// ```rust,ignore
+    /// if !visitor.visit_simple_selector(&self.some_simple_selector) {
+    ///     return false;
+    /// }
+    /// if !self.some_component.visit(visitor) {
+    ///     return false;
+    /// }
+    /// true
+    /// ```
+    pub fn visit<V>(&self, visitor: &mut V) -> bool
+    where
+        V: SelectorVisitor<Impl = Impl>,
+    {
+        let mut current = self.iter();
+        let mut combinator = None;
+        loop {
+            if !visitor.visit_complex_selector(combinator) {
+                return false;
+            }
+
+            for selector in &mut current {
+                if !selector.visit(visitor) {
+                    return false;
+                }
+            }
+
+            combinator = current.next_sequence();
+            if combinator.is_none() {
+                break;
+            }
+        }
+
+        true
+    }
+}
+
+#[derive(Clone)]
+pub struct SelectorIter<'a, Impl: 'a + SelectorImpl> {
+    iter: slice::Iter<'a, Component<Impl>>,
+    next_combinator: Option<Combinator>,
+}
+
+impl<'a, Impl: 'a + SelectorImpl> SelectorIter<'a, Impl> {
+    /// Prepares this iterator to point to the next sequence to the left,
+    /// returning the combinator if the sequence was found.
+    #[inline]
+    pub fn next_sequence(&mut self) -> Option<Combinator> {
+        self.next_combinator.take()
+    }
+
+    /// Whether this selector is a featureless host selector, with no
+    /// combinators to the left.
+    #[inline]
+    pub(crate) fn is_featureless_host_selector(&mut self) -> bool {
+        self.selector_length() > 0 &&
+            self.all(|component| component.is_host()) &&
+            self.next_sequence().is_none()
+    }
+
+    #[inline]
+    pub(crate) fn matches_for_stateless_pseudo_element(&mut self) -> bool {
+        let first = match self.next() {
+            Some(c) => c,
+            // Note that this is the common path that we keep inline: the
+            // pseudo-element not having anything to its right.
+            None => return true,
+        };
+        self.matches_for_stateless_pseudo_element_internal(first)
+    }
+
+    #[inline(never)]
+    fn matches_for_stateless_pseudo_element_internal(&mut self, first: &Component<Impl>) -> bool {
+        if !first.matches_for_stateless_pseudo_element() {
+            return false;
+        }
+        for component in self {
+            // The only other parser-allowed Components in this sequence are
+            // state pseudo-classes, or one of the other things that can contain
+            // them.
+            if !component.matches_for_stateless_pseudo_element() {
+                return false;
+            }
+        }
+        true
+    }
+
+    /// Returns remaining count of the simple selectors and combinators in the Selector.
+    #[inline]
+    pub fn selector_length(&self) -> usize {
+        self.iter.len()
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for SelectorIter<'a, Impl> {
+    type Item = &'a Component<Impl>;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        debug_assert!(
+            self.next_combinator.is_none(),
+            "You should call next_sequence!"
+        );
+        match *self.iter.next()? {
+            Component::Combinator(c) => {
+                self.next_combinator = Some(c);
+                None
+            },
+            ref x => Some(x),
+        }
+    }
+}
+
+impl<'a, Impl: SelectorImpl> fmt::Debug for SelectorIter<'a, Impl> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let iter = self.iter.clone().rev();
+        for component in iter {
+            component.to_css(f)?
+        }
+        Ok(())
+    }
+}
+
+/// An iterator over all simple selectors belonging to ancestors.
+struct AncestorIter<'a, Impl: 'a + SelectorImpl>(SelectorIter<'a, Impl>);
+impl<'a, Impl: 'a + SelectorImpl> AncestorIter<'a, Impl> {
+    /// Creates an AncestorIter. The passed-in iterator is assumed to point to
+    /// the beginning of the child sequence, which will be skipped.
+    fn new(inner: SelectorIter<'a, Impl>) -> Self {
+        let mut result = AncestorIter(inner);
+        result.skip_until_ancestor();
+        result
+    }
+
+    /// Skips a sequence of simple selectors and all subsequent sequences until
+    /// a non-pseudo-element ancestor combinator is reached.
+    fn skip_until_ancestor(&mut self) {
+        loop {
+            while self.0.next().is_some() {}
+            // If this is ever changed to stop at the "pseudo-element"
+            // combinator, we will need to fix the way we compute hashes for
+            // revalidation selectors.
+            if self.0.next_sequence().map_or(true, |x| {
+                matches!(x, Combinator::Child | Combinator::Descendant)
+            }) {
+                break;
+            }
+        }
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for AncestorIter<'a, Impl> {
+    type Item = &'a Component<Impl>;
+    fn next(&mut self) -> Option<Self::Item> {
+        // Grab the next simple selector in the sequence if available.
+        let next = self.0.next();
+        if next.is_some() {
+            return next;
+        }
+
+        // See if there are more sequences. If so, skip any non-ancestor sequences.
+        if let Some(combinator) = self.0.next_sequence() {
+            if !matches!(combinator, Combinator::Child | Combinator::Descendant) {
+                self.skip_until_ancestor();
+            }
+        }
+
+        self.0.next()
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, ToShmem)]
+pub enum Combinator {
+    Child,        //  >
+    Descendant,   // space
+    NextSibling,  // +
+    LaterSibling, // ~
+    /// A dummy combinator we use to the left of pseudo-elements.
+    ///
+    /// It serializes as the empty string, and acts effectively as a child
+    /// combinator in most cases.  If we ever actually start using a child
+    /// combinator for this, we will need to fix up the way hashes are computed
+    /// for revalidation selectors.
+    PseudoElement,
+    /// Another combinator used for ::slotted(), which represent the jump from
+    /// a node to its assigned slot.
+    SlotAssignment,
+    /// Another combinator used for `::part()`, which represents the jump from
+    /// the part to the containing shadow host.
+    Part,
+}
+
+impl Combinator {
+    /// Returns true if this combinator is a child or descendant combinator.
+    #[inline]
+    pub fn is_ancestor(&self) -> bool {
+        matches!(
+            *self,
+            Combinator::Child |
+                Combinator::Descendant |
+                Combinator::PseudoElement |
+                Combinator::SlotAssignment
+        )
+    }
+
+    /// Returns true if this combinator is a pseudo-element combinator.
+    #[inline]
+    pub fn is_pseudo_element(&self) -> bool {
+        matches!(*self, Combinator::PseudoElement)
+    }
+
+    /// Returns true if this combinator is a next- or later-sibling combinator.
+    #[inline]
+    pub fn is_sibling(&self) -> bool {
+        matches!(*self, Combinator::NextSibling | Combinator::LaterSibling)
+    }
+}
+
+/// An enum for the different types of :nth- pseudoclasses
+#[derive(Copy, Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub enum NthType {
+    Child,
+    LastChild,
+    OnlyChild,
+    OfType,
+    LastOfType,
+    OnlyOfType,
+}
+
+impl NthType {
+    pub fn is_only(self) -> bool {
+        self == Self::OnlyChild || self == Self::OnlyOfType
+    }
+
+    pub fn is_of_type(self) -> bool {
+        self == Self::OfType || self == Self::LastOfType || self == Self::OnlyOfType
+    }
+
+    pub fn is_from_end(self) -> bool {
+        self == Self::LastChild || self == Self::LastOfType
+    }
+}
+
+/// The properties that comprise an :nth- pseudoclass as of Selectors 3 (e.g.,
+/// nth-child(An+B)).
+/// https://www.w3.org/TR/selectors-3/#nth-child-pseudo
+#[derive(Copy, Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct NthSelectorData {
+    pub ty: NthType,
+    pub is_function: bool,
+    pub a: i32,
+    pub b: i32,
+}
+
+impl NthSelectorData {
+    /// Returns selector data for :only-{child,of-type}
+    #[inline]
+    pub const fn only(of_type: bool) -> Self {
+        Self {
+            ty: if of_type {
+                NthType::OnlyOfType
+            } else {
+                NthType::OnlyChild
+            },
+            is_function: false,
+            a: 0,
+            b: 1,
+        }
+    }
+
+    /// Returns selector data for :first-{child,of-type}
+    #[inline]
+    pub const fn first(of_type: bool) -> Self {
+        Self {
+            ty: if of_type {
+                NthType::OfType
+            } else {
+                NthType::Child
+            },
+            is_function: false,
+            a: 0,
+            b: 1,
+        }
+    }
+
+    /// Returns selector data for :last-{child,of-type}
+    #[inline]
+    pub const fn last(of_type: bool) -> Self {
+        Self {
+            ty: if of_type {
+                NthType::LastOfType
+            } else {
+                NthType::LastChild
+            },
+            is_function: false,
+            a: 0,
+            b: 1,
+        }
+    }
+
+    /// Writes the beginning of the selector.
+    #[inline]
+    fn write_start<W: fmt::Write>(&self, dest: &mut W) -> fmt::Result {
+        dest.write_str(match self.ty {
+            NthType::Child if self.is_function => ":nth-child(",
+            NthType::Child => ":first-child",
+            NthType::LastChild if self.is_function => ":nth-last-child(",
+            NthType::LastChild => ":last-child",
+            NthType::OfType if self.is_function => ":nth-of-type(",
+            NthType::OfType => ":first-of-type",
+            NthType::LastOfType if self.is_function => ":nth-last-of-type(",
+            NthType::LastOfType => ":last-of-type",
+            NthType::OnlyChild => ":only-child",
+            NthType::OnlyOfType => ":only-of-type",
+        })
+    }
+
+    /// Serialize <an+b> (part of the CSS Syntax spec, but currently only used here).
+    /// <https://drafts.csswg.org/css-syntax-3/#serialize-an-anb-value>
+    #[inline]
+    fn write_affine<W: fmt::Write>(&self, dest: &mut W) -> fmt::Result {
+        match (self.a, self.b) {
+            (0, 0) => dest.write_char('0'),
+
+            (1, 0) => dest.write_char('n'),
+            (-1, 0) => dest.write_str("-n"),
+            (_, 0) => write!(dest, "{}n", self.a),
+
+            (0, _) => write!(dest, "{}", self.b),
+            (1, _) => write!(dest, "n{:+}", self.b),
+            (-1, _) => write!(dest, "-n{:+}", self.b),
+            (_, _) => write!(dest, "{}n{:+}", self.a, self.b),
+        }
+    }
+}
+
+/// The properties that comprise an :nth- pseudoclass as of Selectors 4 (e.g.,
+/// nth-child(An+B [of S]?)).
+/// https://www.w3.org/TR/selectors-4/#nth-child-pseudo
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct NthOfSelectorData<Impl: SelectorImpl>(
+    #[shmem(field_bound)] ThinArc<NthSelectorData, Selector<Impl>>,
+);
+
+impl<Impl: SelectorImpl> NthOfSelectorData<Impl> {
+    /// Returns selector data for :nth-{,last-}{child,of-type}(An+B [of S])
+    #[inline]
+    pub fn new(nth_data: &NthSelectorData, mut selectors: SelectorList<Impl>) -> Self {
+        Self(ThinArc::from_header_and_iter(
+            *nth_data,
+            selectors.0.drain(..),
+        ))
+    }
+
+    /// Returns the An+B part of the selector
+    #[inline]
+    pub fn nth_data(&self) -> &NthSelectorData {
+        &self.0.header.header
+    }
+
+    /// Returns the selector list part of the selector
+    #[inline]
+    pub fn selectors(&self) -> &[Selector<Impl>] {
+        &self.0.slice
+    }
+}
+
+/// A CSS simple selector or combinator. We store both in the same enum for
+/// optimal packing and cache performance, see [1].
+///
+/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1357973
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub enum Component<Impl: SelectorImpl> {
+    LocalName(LocalName<Impl>),
+
+    ID(#[shmem(field_bound)] Impl::Identifier),
+    Class(#[shmem(field_bound)] Impl::Identifier),
+
+    AttributeInNoNamespaceExists {
+        #[shmem(field_bound)]
+        local_name: Impl::LocalName,
+        local_name_lower: Impl::LocalName,
+    },
+    // Used only when local_name is already lowercase.
+    AttributeInNoNamespace {
+        local_name: Impl::LocalName,
+        operator: AttrSelectorOperator,
+        #[shmem(field_bound)]
+        value: Impl::AttrValue,
+        case_sensitivity: ParsedCaseSensitivity,
+        never_matches: bool,
+    },
+    // Use a Box in the less common cases with more data to keep size_of::<Component>() small.
+    AttributeOther(Box<AttrSelectorWithOptionalNamespace<Impl>>),
+
+    ExplicitUniversalType,
+    ExplicitAnyNamespace,
+
+    ExplicitNoNamespace,
+    DefaultNamespace(#[shmem(field_bound)] Impl::NamespaceUrl),
+    Namespace(
+        #[shmem(field_bound)] Impl::NamespacePrefix,
+        #[shmem(field_bound)] Impl::NamespaceUrl,
+    ),
+
+    /// Pseudo-classes
+    Negation(Box<[Selector<Impl>]>),
+    Root,
+    Empty,
+    Scope,
+    Nth(NthSelectorData),
+    NthOf(NthOfSelectorData<Impl>),
+    NonTSPseudoClass(#[shmem(field_bound)] Impl::NonTSPseudoClass),
+    /// The ::slotted() pseudo-element:
+    ///
+    /// https://drafts.csswg.org/css-scoping/#slotted-pseudo
+    ///
+    /// The selector here is a compound selector, that is, no combinators.
+    ///
+    /// NOTE(emilio): This should support a list of selectors, but as of this
+    /// writing no other browser does, and that allows them to put ::slotted()
+    /// in the rule hash, so we do that too.
+    ///
+    /// See https://github.com/w3c/csswg-drafts/issues/2158
+    Slotted(Selector<Impl>),
+    /// The `::part` pseudo-element.
+    ///   https://drafts.csswg.org/css-shadow-parts/#part
+    Part(#[shmem(field_bound)] Box<[Impl::Identifier]>),
+    /// The `:host` pseudo-class:
+    ///
+    /// https://drafts.csswg.org/css-scoping/#host-selector
+    ///
+    /// NOTE(emilio): This should support a list of selectors, but as of this
+    /// writing no other browser does, and that allows them to put :host()
+    /// in the rule hash, so we do that too.
+    ///
+    /// See https://github.com/w3c/csswg-drafts/issues/2158
+    Host(Option<Selector<Impl>>),
+    /// The `:where` pseudo-class.
+    ///
+    /// https://drafts.csswg.org/selectors/#zero-matches
+    ///
+    /// The inner argument is conceptually a SelectorList, but we move the
+    /// selectors to the heap to keep Component small.
+    Where(Box<[Selector<Impl>]>),
+    /// The `:is` pseudo-class.
+    ///
+    /// https://drafts.csswg.org/selectors/#matches-pseudo
+    ///
+    /// Same comment as above re. the argument.
+    Is(Box<[Selector<Impl>]>),
+    /// The `:has` pseudo-class.
+    ///
+    /// https://drafts.csswg.org/selectors/#has-pseudo
+    ///
+    /// Same comment as above re. the argument.
+    Has(Box<[Selector<Impl>]>),
+    /// An implementation-dependent pseudo-element selector.
+    PseudoElement(#[shmem(field_bound)] Impl::PseudoElement),
+
+    Combinator(Combinator),
+}
+
+impl<Impl: SelectorImpl> Component<Impl> {
+    /// Returns true if this is a combinator.
+    #[inline]
+    pub fn is_combinator(&self) -> bool {
+        matches!(*self, Component::Combinator(_))
+    }
+
+    /// Returns true if this is a :host() selector.
+    #[inline]
+    pub fn is_host(&self) -> bool {
+        matches!(*self, Component::Host(..))
+    }
+
+    /// Returns the value as a combinator if applicable, None otherwise.
+    pub fn as_combinator(&self) -> Option<Combinator> {
+        match *self {
+            Component::Combinator(c) => Some(c),
+            _ => None,
+        }
+    }
+
+    /// Whether this component is valid after a pseudo-element. Only intended
+    /// for sanity-checking.
+    pub fn maybe_allowed_after_pseudo_element(&self) -> bool {
+        match *self {
+            Component::NonTSPseudoClass(..) => true,
+            Component::Negation(ref selectors) |
+            Component::Is(ref selectors) |
+            Component::Where(ref selectors) => selectors.iter().all(|selector| {
+                selector
+                    .iter_raw_match_order()
+                    .all(|c| c.maybe_allowed_after_pseudo_element())
+            }),
+            _ => false,
+        }
+    }
+
+    /// Whether a given selector should match for stateless pseudo-elements.
+    ///
+    /// This is a bit subtle: Only selectors that return true in
+    /// `maybe_allowed_after_pseudo_element` should end up here, and
+    /// `NonTSPseudoClass` never matches (as it is a stateless pseudo after
+    /// all).
+    fn matches_for_stateless_pseudo_element(&self) -> bool {
+        debug_assert!(
+            self.maybe_allowed_after_pseudo_element(),
+            "Someone messed up pseudo-element parsing: {:?}",
+            *self
+        );
+        match *self {
+            Component::Negation(ref selectors) => !selectors.iter().all(|selector| {
+                selector
+                    .iter_raw_match_order()
+                    .all(|c| c.matches_for_stateless_pseudo_element())
+            }),
+            Component::Is(ref selectors) | Component::Where(ref selectors) => {
+                selectors.iter().any(|selector| {
+                    selector
+                        .iter_raw_match_order()
+                        .all(|c| c.matches_for_stateless_pseudo_element())
+                })
+            },
+            _ => false,
+        }
+    }
+
+    pub fn visit<V>(&self, visitor: &mut V) -> bool
+    where
+        V: SelectorVisitor<Impl = Impl>,
+    {
+        use self::Component::*;
+        if !visitor.visit_simple_selector(self) {
+            return false;
+        }
+
+        match *self {
+            Slotted(ref selector) => {
+                if !selector.visit(visitor) {
+                    return false;
+                }
+            },
+            Host(Some(ref selector)) => {
+                if !selector.visit(visitor) {
+                    return false;
+                }
+            },
+            AttributeInNoNamespaceExists {
+                ref local_name,
+                ref local_name_lower,
+            } => {
+                if !visitor.visit_attribute_selector(
+                    &NamespaceConstraint::Specific(&namespace_empty_string::<Impl>()),
+                    local_name,
+                    local_name_lower,
+                ) {
+                    return false;
+                }
+            },
+            AttributeInNoNamespace {
+                ref local_name,
+                never_matches,
+                ..
+            } if !never_matches => {
+                if !visitor.visit_attribute_selector(
+                    &NamespaceConstraint::Specific(&namespace_empty_string::<Impl>()),
+                    local_name,
+                    local_name,
+                ) {
+                    return false;
+                }
+            },
+            AttributeOther(ref attr_selector) if !attr_selector.never_matches => {
+                let empty_string;
+                let namespace = match attr_selector.namespace() {
+                    Some(ns) => ns,
+                    None => {
+                        empty_string = crate::parser::namespace_empty_string::<Impl>();
+                        NamespaceConstraint::Specific(&empty_string)
+                    },
+                };
+                if !visitor.visit_attribute_selector(
+                    &namespace,
+                    &attr_selector.local_name,
+                    &attr_selector.local_name_lower,
+                ) {
+                    return false;
+                }
+            },
+
+            NonTSPseudoClass(ref pseudo_class) => {
+                if !pseudo_class.visit(visitor) {
+                    return false;
+                }
+            },
+
+            Negation(ref list) | Is(ref list) | Where(ref list) => {
+                if !visitor.visit_selector_list(&list) {
+                    return false;
+                }
+            },
+            NthOf(ref nth_of_data) => {
+                if !visitor.visit_selector_list(nth_of_data.selectors()) {
+                    return false;
+                }
+            },
+            _ => {},
+        }
+
+        true
+    }
+}
+
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+#[shmem(no_bounds)]
+pub struct LocalName<Impl: SelectorImpl> {
+    #[shmem(field_bound)]
+    pub name: Impl::LocalName,
+    pub lower_name: Impl::LocalName,
+}
+
+impl<Impl: SelectorImpl> Debug for Selector<Impl> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("Selector(")?;
+        self.to_css(f)?;
+        write!(f, ", specificity = 0x{:x})", self.specificity())
+    }
+}
+
+impl<Impl: SelectorImpl> Debug for Component<Impl> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.to_css(f)
+    }
+}
+impl<Impl: SelectorImpl> Debug for AttrSelectorWithOptionalNamespace<Impl> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.to_css(f)
+    }
+}
+impl<Impl: SelectorImpl> Debug for LocalName<Impl> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.to_css(f)
+    }
+}
+
+fn serialize_selector_list<'a, Impl, I, W>(iter: I, dest: &mut W) -> fmt::Result
+where
+    Impl: SelectorImpl,
+    I: Iterator<Item = &'a Selector<Impl>>,
+    W: fmt::Write,
+{
+    let mut first = true;
+    for selector in iter {
+        if !first {
+            dest.write_str(", ")?;
+        }
+        first = false;
+        selector.to_css(dest)?;
+    }
+    Ok(())
+}
+
+impl<Impl: SelectorImpl> ToCss for SelectorList<Impl> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        serialize_selector_list(self.0.iter(), dest)
+    }
+}
+
+impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        // Compound selectors invert the order of their contents, so we need to
+        // undo that during serialization.
+        //
+        // This two-iterator strategy involves walking over the selector twice.
+        // We could do something more clever, but selector serialization probably
+        // isn't hot enough to justify it, and the stringification likely
+        // dominates anyway.
+        //
+        // NB: A parse-order iterator is a Rev<>, which doesn't expose as_slice(),
+        // which we need for |split|. So we split by combinators on a match-order
+        // sequence and then reverse.
+
+        let mut combinators = self
+            .iter_raw_match_order()
+            .rev()
+            .filter_map(|x| x.as_combinator());
+        let compound_selectors = self
+            .iter_raw_match_order()
+            .as_slice()
+            .split(|x| x.is_combinator())
+            .rev();
+
+        let mut combinators_exhausted = false;
+        for compound in compound_selectors {
+            debug_assert!(!combinators_exhausted);
+
+            // https://drafts.csswg.org/cssom/#serializing-selectors
+            if compound.is_empty() {
+                continue;
+            }
+
+            // 1. If there is only one simple selector in the compound selectors
+            //    which is a universal selector, append the result of
+            //    serializing the universal selector to s.
+            //
+            // Check if `!compound.empty()` first--this can happen if we have
+            // something like `... > ::before`, because we store `>` and `::`
+            // both as combinators internally.
+            //
+            // If we are in this case, after we have serialized the universal
+            // selector, we skip Step 2 and continue with the algorithm.
+            let (can_elide_namespace, first_non_namespace) = match compound[0] {
+                Component::ExplicitAnyNamespace |
+                Component::ExplicitNoNamespace |
+                Component::Namespace(..) => (false, 1),
+                Component::DefaultNamespace(..) => (true, 1),
+                _ => (true, 0),
+            };
+            let mut perform_step_2 = true;
+            let next_combinator = combinators.next();
+            if first_non_namespace == compound.len() - 1 {
+                match (next_combinator, &compound[first_non_namespace]) {
+                    // We have to be careful here, because if there is a
+                    // pseudo element "combinator" there isn't really just
+                    // the one simple selector. Technically this compound
+                    // selector contains the pseudo element selector as well
+                    // -- Combinator::PseudoElement, just like
+                    // Combinator::SlotAssignment, don't exist in the
+                    // spec.
+                    (Some(Combinator::PseudoElement), _) |
+                    (Some(Combinator::SlotAssignment), _) => (),
+                    (_, &Component::ExplicitUniversalType) => {
+                        // Iterate over everything so we serialize the namespace
+                        // too.
+                        for simple in compound.iter() {
+                            simple.to_css(dest)?;
+                        }
+                        // Skip step 2, which is an "otherwise".
+                        perform_step_2 = false;
+                    },
+                    _ => (),
+                }
+            }
+
+            // 2. Otherwise, for each simple selector in the compound selectors
+            //    that is not a universal selector of which the namespace prefix
+            //    maps to a namespace that is not the default namespace
+            //    serialize the simple selector and append the result to s.
+            //
+            // See https://github.com/w3c/csswg-drafts/issues/1606, which is
+            // proposing to change this to match up with the behavior asserted
+            // in cssom/serialize-namespaced-type-selectors.html, which the
+            // following code tries to match.
+            if perform_step_2 {
+                for simple in compound.iter() {
+                    if let Component::ExplicitUniversalType = *simple {
+                        // Can't have a namespace followed by a pseudo-element
+                        // selector followed by a universal selector in the same
+                        // compound selector, so we don't have to worry about the
+                        // real namespace being in a different `compound`.
+                        if can_elide_namespace {
+                            continue;
+                        }
+                    }
+                    simple.to_css(dest)?;
+                }
+            }
+
+            // 3. If this is not the last part of the chain of the selector
+            //    append a single SPACE (U+0020), followed by the combinator
+            //    ">", "+", "~", ">>", "||", as appropriate, followed by another
+            //    single SPACE (U+0020) if the combinator was not whitespace, to
+            //    s.
+            match next_combinator {
+                Some(c) => c.to_css(dest)?,
+                None => combinators_exhausted = true,
+            };
+
+            // 4. If this is the last part of the chain of the selector and
+            //    there is a pseudo-element, append "::" followed by the name of
+            //    the pseudo-element, to s.
+            //
+            // (we handle this above)
+        }
+
+        Ok(())
+    }
+}
+
+impl ToCss for Combinator {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        match *self {
+            Combinator::Child => dest.write_str(" > "),
+            Combinator::Descendant => dest.write_str(" "),
+            Combinator::NextSibling => dest.write_str(" + "),
+            Combinator::LaterSibling => dest.write_str(" ~ "),
+            Combinator::PseudoElement | Combinator::Part | Combinator::SlotAssignment => Ok(()),
+        }
+    }
+}
+
+impl<Impl: SelectorImpl> ToCss for Component<Impl> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        use self::Component::*;
+
+        match *self {
+            Combinator(ref c) => c.to_css(dest),
+            Slotted(ref selector) => {
+                dest.write_str("::slotted(")?;
+                selector.to_css(dest)?;
+                dest.write_char(')')
+            },
+            Part(ref part_names) => {
+                dest.write_str("::part(")?;
+                for (i, name) in part_names.iter().enumerate() {
+                    if i != 0 {
+                        dest.write_char(' ')?;
+                    }
+                    name.to_css(dest)?;
+                }
+                dest.write_char(')')
+            },
+            PseudoElement(ref p) => p.to_css(dest),
+            ID(ref s) => {
+                dest.write_char('#')?;
+                s.to_css(dest)
+            },
+            Class(ref s) => {
+                dest.write_char('.')?;
+                s.to_css(dest)
+            },
+            LocalName(ref s) => s.to_css(dest),
+            ExplicitUniversalType => dest.write_char('*'),
+
+            DefaultNamespace(_) => Ok(()),
+            ExplicitNoNamespace => dest.write_char('|'),
+            ExplicitAnyNamespace => dest.write_str("*|"),
+            Namespace(ref prefix, _) => {
+                prefix.to_css(dest)?;
+                dest.write_char('|')
+            },
+
+            AttributeInNoNamespaceExists { ref local_name, .. } => {
+                dest.write_char('[')?;
+                local_name.to_css(dest)?;
+                dest.write_char(']')
+            },
+            AttributeInNoNamespace {
+                ref local_name,
+                operator,
+                ref value,
+                case_sensitivity,
+                ..
+            } => {
+                dest.write_char('[')?;
+                local_name.to_css(dest)?;
+                operator.to_css(dest)?;
+                dest.write_char('"')?;
+                value.to_css(dest)?;
+                dest.write_char('"')?;
+                match case_sensitivity {
+                    ParsedCaseSensitivity::CaseSensitive |
+                    ParsedCaseSensitivity::AsciiCaseInsensitiveIfInHtmlElementInHtmlDocument => {},
+                    ParsedCaseSensitivity::AsciiCaseInsensitive => dest.write_str(" i")?,
+                    ParsedCaseSensitivity::ExplicitCaseSensitive => dest.write_str(" s")?,
+                }
+                dest.write_char(']')
+            },
+            AttributeOther(ref attr_selector) => attr_selector.to_css(dest),
+
+            // Pseudo-classes
+            Root => dest.write_str(":root"),
+            Empty => dest.write_str(":empty"),
+            Scope => dest.write_str(":scope"),
+            Host(ref selector) => {
+                dest.write_str(":host")?;
+                if let Some(ref selector) = *selector {
+                    dest.write_char('(')?;
+                    selector.to_css(dest)?;
+                    dest.write_char(')')?;
+                }
+                Ok(())
+            },
+            Nth(ref nth_data) => {
+                nth_data.write_start(dest)?;
+                if nth_data.is_function {
+                    nth_data.write_affine(dest)?;
+                    dest.write_char(')')?;
+                }
+                Ok(())
+            },
+            NthOf(ref nth_of_data) => {
+                let nth_data = nth_of_data.nth_data();
+                nth_data.write_start(dest)?;
+                debug_assert!(
+                    nth_data.is_function,
+                    "A selector must be a function to hold An+B notation"
+                );
+                nth_data.write_affine(dest)?;
+                debug_assert!(
+                    matches!(nth_data.ty, NthType::Child | NthType::LastChild),
+                    "Only :nth-child or :nth-last-child can be of a selector list"
+                );
+                debug_assert!(
+                    !nth_of_data.selectors().is_empty(),
+                    "The selector list should not be empty"
+                );
+                dest.write_str(" of ")?;
+                serialize_selector_list(nth_of_data.selectors().iter(), dest)?;
+                dest.write_char(')')
+            },
+            Is(ref list) | Where(ref list) | Negation(ref list) | Has(ref list) => {
+                match *self {
+                    Where(..) => dest.write_str(":where(")?,
+                    Is(..) => dest.write_str(":is(")?,
+                    Negation(..) => dest.write_str(":not(")?,
+                    Has(..) => dest.write_str(":has(")?,
+                    _ => unreachable!(),
+                }
+                serialize_selector_list(list.iter(), dest)?;
+                dest.write_str(")")
+            },
+            NonTSPseudoClass(ref pseudo) => pseudo.to_css(dest),
+        }
+    }
+}
+
+impl<Impl: SelectorImpl> ToCss for AttrSelectorWithOptionalNamespace<Impl> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        dest.write_char('[')?;
+        match self.namespace {
+            Some(NamespaceConstraint::Specific((ref prefix, _))) => {
+                prefix.to_css(dest)?;
+                dest.write_char('|')?
+            },
+            Some(NamespaceConstraint::Any) => dest.write_str("*|")?,
+            None => {},
+        }
+        self.local_name.to_css(dest)?;
+        match self.operation {
+            ParsedAttrSelectorOperation::Exists => {},
+            ParsedAttrSelectorOperation::WithValue {
+                operator,
+                case_sensitivity,
+                ref expected_value,
+            } => {
+                operator.to_css(dest)?;
+                dest.write_char('"')?;
+                expected_value.to_css(dest)?;
+                dest.write_char('"')?;
+                match case_sensitivity {
+                    ParsedCaseSensitivity::CaseSensitive |
+                    ParsedCaseSensitivity::AsciiCaseInsensitiveIfInHtmlElementInHtmlDocument => {},
+                    ParsedCaseSensitivity::AsciiCaseInsensitive => dest.write_str(" i")?,
+                    ParsedCaseSensitivity::ExplicitCaseSensitive => dest.write_str(" s")?,
+                }
+            },
+        }
+        dest.write_char(']')
+    }
+}
+
+impl<Impl: SelectorImpl> ToCss for LocalName<Impl> {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        self.name.to_css(dest)
+    }
+}
+
+/// Build up a Selector.
+/// selector : simple_selector_sequence [ combinator simple_selector_sequence ]* ;
+///
+/// `Err` means invalid selector.
+fn parse_selector<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    mut state: SelectorParsingState,
+) -> Result<Selector<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    let mut builder = SelectorBuilder::default();
+
+    let mut has_pseudo_element = false;
+    let mut slotted = false;
+    let mut part = false;
+    'outer_loop: loop {
+        // Parse a sequence of simple selectors.
+        let empty = parse_compound_selector(parser, &mut state, input, &mut builder)?;
+        if empty {
+            return Err(input.new_custom_error(if builder.has_combinators() {
+                SelectorParseErrorKind::DanglingCombinator
+            } else {
+                SelectorParseErrorKind::EmptySelector
+            }));
+        }
+
+        if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+            has_pseudo_element = state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+            slotted = state.intersects(SelectorParsingState::AFTER_SLOTTED);
+            part = state.intersects(SelectorParsingState::AFTER_PART);
+            debug_assert!(has_pseudo_element || slotted || part);
+            break;
+        }
+
+        // Parse a combinator.
+        let combinator;
+        let mut any_whitespace = false;
+        loop {
+            let before_this_token = input.state();
+            match input.next_including_whitespace() {
+                Err(_e) => break 'outer_loop,
+                Ok(&Token::WhiteSpace(_)) => any_whitespace = true,
+                Ok(&Token::Delim('>')) => {
+                    combinator = Combinator::Child;
+                    break;
+                },
+                Ok(&Token::Delim('+')) => {
+                    combinator = Combinator::NextSibling;
+                    break;
+                },
+                Ok(&Token::Delim('~')) => {
+                    combinator = Combinator::LaterSibling;
+                    break;
+                },
+                Ok(_) => {
+                    input.reset(&before_this_token);
+                    if any_whitespace {
+                        combinator = Combinator::Descendant;
+                        break;
+                    } else {
+                        break 'outer_loop;
+                    }
+                },
+            }
+        }
+
+        if !state.allows_combinators() {
+            return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+        }
+
+        builder.push_combinator(combinator);
+    }
+
+    Ok(Selector(builder.build(has_pseudo_element, slotted, part)))
+}
+
+impl<Impl: SelectorImpl> Selector<Impl> {
+    /// Parse a selector, without any pseudo-element.
+    #[inline]
+    pub fn parse<'i, 't, P>(
+        parser: &P,
+        input: &mut CssParser<'i, 't>,
+    ) -> Result<Self, ParseError<'i, P::Error>>
+    where
+        P: Parser<'i, Impl = Impl>,
+    {
+        parse_selector(parser, input, SelectorParsingState::empty())
+    }
+}
+
+/// * `Err(())`: Invalid selector, abort
+/// * `Ok(false)`: Not a type selector, could be something else. `input` was not consumed.
+/// * `Ok(true)`: Length 0 (`*|*`), 1 (`*|E` or `ns|*`) or 2 (`|E` or `ns|E`)
+fn parse_type_selector<'i, 't, P, Impl, S>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+    sink: &mut S,
+) -> Result<bool, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+    S: Push<Component<Impl>>,
+{
+    match parse_qualified_name(parser, input, /* in_attr_selector = */ false) {
+        Err(ParseError {
+            kind: ParseErrorKind::Basic(BasicParseErrorKind::EndOfInput),
+            ..
+        }) |
+        Ok(OptionalQName::None(_)) => Ok(false),
+        Ok(OptionalQName::Some(namespace, local_name)) => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            match namespace {
+                QNamePrefix::ImplicitAnyNamespace => {},
+                QNamePrefix::ImplicitDefaultNamespace(url) => {
+                    sink.push(Component::DefaultNamespace(url))
+                },
+                QNamePrefix::ExplicitNamespace(prefix, url) => {
+                    sink.push(match parser.default_namespace() {
+                        Some(ref default_url) if url == *default_url => {
+                            Component::DefaultNamespace(url)
+                        },
+                        _ => Component::Namespace(prefix, url),
+                    })
+                },
+                QNamePrefix::ExplicitNoNamespace => sink.push(Component::ExplicitNoNamespace),
+                QNamePrefix::ExplicitAnyNamespace => {
+                    match parser.default_namespace() {
+                        // Element type selectors that have no namespace
+                        // component (no namespace separator) represent elements
+                        // without regard to the element's namespace (equivalent
+                        // to "*|") unless a default namespace has been declared
+                        // for namespaced selectors (e.g. in CSS, in the style
+                        // sheet). If a default namespace has been declared,
+                        // such selectors will represent only elements in the
+                        // default namespace.
+                        // -- Selectors § 6.1.1
+                        // So we'll have this act the same as the
+                        // QNamePrefix::ImplicitAnyNamespace case.
+                        None => {},
+                        Some(_) => sink.push(Component::ExplicitAnyNamespace),
+                    }
+                },
+                QNamePrefix::ImplicitNoNamespace => {
+                    unreachable!() // Not returned with in_attr_selector = false
+                },
+            }
+            match local_name {
+                Some(name) => sink.push(Component::LocalName(LocalName {
+                    lower_name: to_ascii_lowercase(&name).as_ref().into(),
+                    name: name.as_ref().into(),
+                })),
+                None => sink.push(Component::ExplicitUniversalType),
+            }
+            Ok(true)
+        },
+        Err(e) => Err(e),
+    }
+}
+
+#[derive(Debug)]
+enum SimpleSelectorParseResult<Impl: SelectorImpl> {
+    SimpleSelector(Component<Impl>),
+    PseudoElement(Impl::PseudoElement),
+    SlottedPseudo(Selector<Impl>),
+    PartPseudo(Box<[Impl::Identifier]>),
+}
+
+#[derive(Debug)]
+enum QNamePrefix<Impl: SelectorImpl> {
+    ImplicitNoNamespace,                          // `foo` in attr selectors
+    ImplicitAnyNamespace,                         // `foo` in type selectors, without a default ns
+    ImplicitDefaultNamespace(Impl::NamespaceUrl), // `foo` in type selectors, with a default ns
+    ExplicitNoNamespace,                          // `|foo`
+    ExplicitAnyNamespace,                         // `*|foo`
+    ExplicitNamespace(Impl::NamespacePrefix, Impl::NamespaceUrl), // `prefix|foo`
+}
+
+enum OptionalQName<'i, Impl: SelectorImpl> {
+    Some(QNamePrefix<Impl>, Option<CowRcStr<'i>>),
+    None(Token<'i>),
+}
+
+/// * `Err(())`: Invalid selector, abort
+/// * `Ok(None(token))`: Not a simple selector, could be something else. `input` was not consumed,
+///                      but the token is still returned.
+/// * `Ok(Some(namespace, local_name))`: `None` for the local name means a `*` universal selector
+fn parse_qualified_name<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    in_attr_selector: bool,
+) -> Result<OptionalQName<'i, Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    let default_namespace = |local_name| {
+        let namespace = match parser.default_namespace() {
+            Some(url) => QNamePrefix::ImplicitDefaultNamespace(url),
+            None => QNamePrefix::ImplicitAnyNamespace,
+        };
+        Ok(OptionalQName::Some(namespace, local_name))
+    };
+
+    let explicit_namespace = |input: &mut CssParser<'i, 't>, namespace| {
+        let location = input.current_source_location();
+        match input.next_including_whitespace() {
+            Ok(&Token::Delim('*')) if !in_attr_selector => Ok(OptionalQName::Some(namespace, None)),
+            Ok(&Token::Ident(ref local_name)) => {
+                Ok(OptionalQName::Some(namespace, Some(local_name.clone())))
+            },
+            Ok(t) if in_attr_selector => {
+                let e = SelectorParseErrorKind::InvalidQualNameInAttr(t.clone());
+                Err(location.new_custom_error(e))
+            },
+            Ok(t) => Err(location.new_custom_error(
+                SelectorParseErrorKind::ExplicitNamespaceUnexpectedToken(t.clone()),
+            )),
+            Err(e) => Err(e.into()),
+        }
+    };
+
+    let start = input.state();
+    match input.next_including_whitespace() {
+        Ok(Token::Ident(value)) => {
+            let value = value.clone();
+            let after_ident = input.state();
+            match input.next_including_whitespace() {
+                Ok(&Token::Delim('|')) => {
+                    let prefix = value.as_ref().into();
+                    let result = parser.namespace_for_prefix(&prefix);
+                    let url = result.ok_or(
+                        after_ident
+                            .source_location()
+                            .new_custom_error(SelectorParseErrorKind::ExpectedNamespace(value)),
+                    )?;
+                    explicit_namespace(input, QNamePrefix::ExplicitNamespace(prefix, url))
+                },
+                _ => {
+                    input.reset(&after_ident);
+                    if in_attr_selector {
+                        Ok(OptionalQName::Some(
+                            QNamePrefix::ImplicitNoNamespace,
+                            Some(value),
+                        ))
+                    } else {
+                        default_namespace(Some(value))
+                    }
+                },
+            }
+        },
+        Ok(Token::Delim('*')) => {
+            let after_star = input.state();
+            match input.next_including_whitespace() {
+                Ok(&Token::Delim('|')) => {
+                    explicit_namespace(input, QNamePrefix::ExplicitAnyNamespace)
+                },
+                _ if !in_attr_selector => {
+                    input.reset(&after_star);
+                    default_namespace(None)
+                },
+                result => {
+                    let t = result?;
+                    Err(after_star
+                        .source_location()
+                        .new_custom_error(SelectorParseErrorKind::ExpectedBarInAttr(t.clone())))
+                },
+            }
+        },
+        Ok(Token::Delim('|')) => explicit_namespace(input, QNamePrefix::ExplicitNoNamespace),
+        Ok(t) => {
+            let t = t.clone();
+            input.reset(&start);
+            Ok(OptionalQName::None(t))
+        },
+        Err(e) => {
+            input.reset(&start);
+            Err(e.into())
+        },
+    }
+}
+
+fn parse_attribute_selector<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    let namespace;
+    let local_name;
+
+    input.skip_whitespace();
+
+    match parse_qualified_name(parser, input, /* in_attr_selector = */ true)? {
+        OptionalQName::None(t) => {
+            return Err(input.new_custom_error(
+                SelectorParseErrorKind::NoQualifiedNameInAttributeSelector(t),
+            ));
+        },
+        OptionalQName::Some(_, None) => unreachable!(),
+        OptionalQName::Some(ns, Some(ln)) => {
+            local_name = ln;
+            namespace = match ns {
+                QNamePrefix::ImplicitNoNamespace | QNamePrefix::ExplicitNoNamespace => None,
+                QNamePrefix::ExplicitNamespace(prefix, url) => {
+                    Some(NamespaceConstraint::Specific((prefix, url)))
+                },
+                QNamePrefix::ExplicitAnyNamespace => Some(NamespaceConstraint::Any),
+                QNamePrefix::ImplicitAnyNamespace | QNamePrefix::ImplicitDefaultNamespace(_) => {
+                    unreachable!() // Not returned with in_attr_selector = true
+                },
+            }
+        },
+    }
+
+    let location = input.current_source_location();
+    let operator = match input.next() {
+        // [foo]
+        Err(_) => {
+            let local_name_lower = to_ascii_lowercase(&local_name).as_ref().into();
+            let local_name = local_name.as_ref().into();
+            if let Some(namespace) = namespace {
+                return Ok(Component::AttributeOther(Box::new(
+                    AttrSelectorWithOptionalNamespace {
+                        namespace: Some(namespace),
+                        local_name,
+                        local_name_lower,
+                        operation: ParsedAttrSelectorOperation::Exists,
+                        never_matches: false,
+                    },
+                )));
+            } else {
+                return Ok(Component::AttributeInNoNamespaceExists {
+                    local_name,
+                    local_name_lower,
+                });
+            }
+        },
+
+        // [foo=bar]
+        Ok(&Token::Delim('=')) => AttrSelectorOperator::Equal,
+        // [foo~=bar]
+        Ok(&Token::IncludeMatch) => AttrSelectorOperator::Includes,
+        // [foo|=bar]
+        Ok(&Token::DashMatch) => AttrSelectorOperator::DashMatch,
+        // [foo^=bar]
+        Ok(&Token::PrefixMatch) => AttrSelectorOperator::Prefix,
+        // [foo*=bar]
+        Ok(&Token::SubstringMatch) => AttrSelectorOperator::Substring,
+        // [foo$=bar]
+        Ok(&Token::SuffixMatch) => AttrSelectorOperator::Suffix,
+        Ok(t) => {
+            return Err(location.new_custom_error(
+                SelectorParseErrorKind::UnexpectedTokenInAttributeSelector(t.clone()),
+            ));
+        },
+    };
+
+    let value = match input.expect_ident_or_string() {
+        Ok(t) => t.clone(),
+        Err(BasicParseError {
+            kind: BasicParseErrorKind::UnexpectedToken(t),
+            location,
+        }) => return Err(location.new_custom_error(SelectorParseErrorKind::BadValueInAttr(t))),
+        Err(e) => return Err(e.into()),
+    };
+    let never_matches = match operator {
+        AttrSelectorOperator::Equal | AttrSelectorOperator::DashMatch => false,
+
+        AttrSelectorOperator::Includes => value.is_empty() || value.contains(SELECTOR_WHITESPACE),
+
+        AttrSelectorOperator::Prefix |
+        AttrSelectorOperator::Substring |
+        AttrSelectorOperator::Suffix => value.is_empty(),
+    };
+
+    let attribute_flags = parse_attribute_flags(input)?;
+
+    let value = value.as_ref().into();
+    let local_name_lower;
+    let local_name_is_ascii_lowercase;
+    let case_sensitivity;
+    {
+        let local_name_lower_cow = to_ascii_lowercase(&local_name);
+        case_sensitivity =
+            attribute_flags.to_case_sensitivity(local_name_lower_cow.as_ref(), namespace.is_some());
+        local_name_lower = local_name_lower_cow.as_ref().into();
+        local_name_is_ascii_lowercase = matches!(local_name_lower_cow, Cow::Borrowed(..));
+    }
+    let local_name = local_name.as_ref().into();
+    if namespace.is_some() || !local_name_is_ascii_lowercase {
+        Ok(Component::AttributeOther(Box::new(
+            AttrSelectorWithOptionalNamespace {
+                namespace,
+                local_name,
+                local_name_lower,
+                never_matches,
+                operation: ParsedAttrSelectorOperation::WithValue {
+                    operator,
+                    case_sensitivity,
+                    expected_value: value,
+                },
+            },
+        )))
+    } else {
+        Ok(Component::AttributeInNoNamespace {
+            local_name,
+            operator,
+            value,
+            case_sensitivity,
+            never_matches,
+        })
+    }
+}
+
+/// An attribute selector can have 's' or 'i' as flags, or no flags at all.
+enum AttributeFlags {
+    // Matching should be case-sensitive ('s' flag).
+    CaseSensitive,
+    // Matching should be case-insensitive ('i' flag).
+    AsciiCaseInsensitive,
+    // No flags.  Matching behavior depends on the name of the attribute.
+    CaseSensitivityDependsOnName,
+}
+
+impl AttributeFlags {
+    fn to_case_sensitivity(self, local_name: &str, have_namespace: bool) -> ParsedCaseSensitivity {
+        match self {
+            AttributeFlags::CaseSensitive => ParsedCaseSensitivity::ExplicitCaseSensitive,
+            AttributeFlags::AsciiCaseInsensitive => ParsedCaseSensitivity::AsciiCaseInsensitive,
+            AttributeFlags::CaseSensitivityDependsOnName => {
+                if !have_namespace &&
+                    include!(concat!(
+                        env!("OUT_DIR"),
+                        "/ascii_case_insensitive_html_attributes.rs"
+                    ))
+                    .contains(local_name)
+                {
+                    ParsedCaseSensitivity::AsciiCaseInsensitiveIfInHtmlElementInHtmlDocument
+                } else {
+                    ParsedCaseSensitivity::CaseSensitive
+                }
+            },
+        }
+    }
+}
+
+fn parse_attribute_flags<'i, 't>(
+    input: &mut CssParser<'i, 't>,
+) -> Result<AttributeFlags, BasicParseError<'i>> {
+    let location = input.current_source_location();
+    let token = match input.next() {
+        Ok(t) => t,
+        Err(..) => {
+            // Selectors spec says language-defined; HTML says it depends on the
+            // exact attribute name.
+            return Ok(AttributeFlags::CaseSensitivityDependsOnName);
+        },
+    };
+
+    let ident = match *token {
+        Token::Ident(ref i) => i,
+        ref other => return Err(location.new_basic_unexpected_token_error(other.clone())),
+    };
+
+    Ok(match_ignore_ascii_case! {
+        ident,
+        "i" => AttributeFlags::AsciiCaseInsensitive,
+        "s" => AttributeFlags::CaseSensitive,
+        _ => return Err(location.new_basic_unexpected_token_error(token.clone())),
+    })
+}
+
+/// Level 3: Parse **one** simple_selector.  (Though we might insert a second
+/// implied "<defaultns>|*" type selector.)
+fn parse_negation<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    let list = SelectorList::parse_with_state(
+        parser,
+        input,
+        state |
+            SelectorParsingState::SKIP_DEFAULT_NAMESPACE |
+            SelectorParsingState::DISALLOW_PSEUDOS,
+        ParseErrorRecovery::DiscardList,
+    )?;
+
+    Ok(Component::Negation(list.0.into_vec().into_boxed_slice()))
+}
+
+/// simple_selector_sequence
+/// : [ type_selector | universal ] [ HASH | class | attrib | pseudo | negation ]*
+/// | [ HASH | class | attrib | pseudo | negation ]+
+///
+/// `Err(())` means invalid selector.
+/// `Ok(true)` is an empty selector
+fn parse_compound_selector<'i, 't, P, Impl>(
+    parser: &P,
+    state: &mut SelectorParsingState,
+    input: &mut CssParser<'i, 't>,
+    builder: &mut SelectorBuilder<Impl>,
+) -> Result<bool, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    input.skip_whitespace();
+
+    let mut empty = true;
+    if parse_type_selector(parser, input, *state, builder)? {
+        empty = false;
+    }
+
+    loop {
+        let result = match parse_one_simple_selector(parser, input, *state)? {
+            None => break,
+            Some(result) => result,
+        };
+
+        if empty {
+            if let Some(url) = parser.default_namespace() {
+                // If there was no explicit type selector, but there is a
+                // default namespace, there is an implicit "<defaultns>|*" type
+                // selector. Except for :host() or :not() / :is() / :where(),
+                // where we ignore it.
+                //
+                // https://drafts.csswg.org/css-scoping/#host-element-in-tree:
+                //
+                //     When considered within its own shadow trees, the shadow
+                //     host is featureless. Only the :host, :host(), and
+                //     :host-context() pseudo-classes are allowed to match it.
+                //
+                // https://drafts.csswg.org/selectors-4/#featureless:
+                //
+                //     A featureless element does not match any selector at all,
+                //     except those it is explicitly defined to match. If a
+                //     given selector is allowed to match a featureless element,
+                //     it must do so while ignoring the default namespace.
+                //
+                // https://drafts.csswg.org/selectors-4/#matches
+                //
+                //     Default namespace declarations do not affect the compound
+                //     selector representing the subject of any selector within
+                //     a :is() pseudo-class, unless that compound selector
+                //     contains an explicit universal selector or type selector.
+                //
+                //     (Similar quotes for :where() / :not())
+                //
+                let ignore_default_ns = state
+                    .intersects(SelectorParsingState::SKIP_DEFAULT_NAMESPACE) ||
+                    matches!(
+                        result,
+                        SimpleSelectorParseResult::SimpleSelector(Component::Host(..))
+                    );
+                if !ignore_default_ns {
+                    builder.push_simple_selector(Component::DefaultNamespace(url));
+                }
+            }
+        }
+
+        empty = false;
+
+        match result {
+            SimpleSelectorParseResult::SimpleSelector(s) => {
+                builder.push_simple_selector(s);
+            },
+            SimpleSelectorParseResult::PartPseudo(part_names) => {
+                state.insert(SelectorParsingState::AFTER_PART);
+                builder.push_combinator(Combinator::Part);
+                builder.push_simple_selector(Component::Part(part_names));
+            },
+            SimpleSelectorParseResult::SlottedPseudo(selector) => {
+                state.insert(SelectorParsingState::AFTER_SLOTTED);
+                builder.push_combinator(Combinator::SlotAssignment);
+                builder.push_simple_selector(Component::Slotted(selector));
+            },
+            SimpleSelectorParseResult::PseudoElement(p) => {
+                state.insert(SelectorParsingState::AFTER_PSEUDO_ELEMENT);
+                if !p.accepts_state_pseudo_classes() {
+                    state.insert(SelectorParsingState::AFTER_NON_STATEFUL_PSEUDO_ELEMENT);
+                }
+                builder.push_combinator(Combinator::PseudoElement);
+                builder.push_simple_selector(Component::PseudoElement(p));
+            },
+        }
+    }
+    Ok(empty)
+}
+
+fn parse_is_where_has<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+    component: impl FnOnce(Box<[Selector<Impl>]>) -> Component<Impl>,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    debug_assert!(parser.parse_is_and_where());
+    // https://drafts.csswg.org/selectors/#matches-pseudo:
+    //
+    //     Pseudo-elements cannot be represented by the matches-any
+    //     pseudo-class; they are not valid within :is().
+    //
+    let inner = SelectorList::parse_with_state(
+        parser,
+        input,
+        state |
+            SelectorParsingState::SKIP_DEFAULT_NAMESPACE |
+            SelectorParsingState::DISALLOW_PSEUDOS,
+        ParseErrorRecovery::IgnoreInvalidSelector,
+    )?;
+    Ok(component(inner.0.into_vec().into_boxed_slice()))
+}
+
+fn parse_functional_pseudo_class<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    name: CowRcStr<'i>,
+    state: SelectorParsingState,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    match_ignore_ascii_case! { &name,
+        "nth-child" => return parse_nth_pseudo_class(parser, input, state, NthType::Child),
+        "nth-of-type" => return parse_nth_pseudo_class(parser, input, state, NthType::OfType),
+        "nth-last-child" => return parse_nth_pseudo_class(parser, input, state, NthType::LastChild),
+        "nth-last-of-type" => return parse_nth_pseudo_class(parser, input, state, NthType::LastOfType),
+        "is" if parser.parse_is_and_where() => return parse_is_where_has(parser, input, state, Component::Is),
+        "where" if parser.parse_is_and_where() => return parse_is_where_has(parser, input, state, Component::Where),
+        "has" if parser.parse_has() => return parse_is_where_has(parser, input, state, Component::Has),
+        "host" => {
+            if !state.allows_tree_structural_pseudo_classes() {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            return Ok(Component::Host(Some(parse_inner_compound_selector(parser, input, state)?)));
+        },
+        "not" => {
+            return parse_negation(parser, input, state)
+        },
+        _ => {}
+    }
+
+    if parser.parse_is_and_where() && parser.is_is_alias(&name) {
+        return parse_is_where_has(parser, input, state, Component::Is);
+    }
+
+    if !state.allows_custom_functional_pseudo_classes() {
+        return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+
+    P::parse_non_ts_functional_pseudo_class(parser, name, input).map(Component::NonTSPseudoClass)
+}
+
+fn parse_nth_pseudo_class<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+    ty: NthType,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    if !state.allows_tree_structural_pseudo_classes() {
+        return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+    let (a, b) = parse_nth(input)?;
+    let nth_data = NthSelectorData {
+        ty,
+        is_function: true,
+        a,
+        b,
+    };
+    if !parser.parse_nth_child_of() || ty.is_of_type() {
+        return Ok(Component::Nth(nth_data));
+    }
+
+    // Try to parse "of <selector-list>".
+    if input.try_parse(|i| i.expect_ident_matching("of")).is_err() {
+        return Ok(Component::Nth(nth_data));
+    }
+    // Whitespace between "of" and the selector list is optional
+    // https://github.com/w3c/csswg-drafts/issues/8285
+    let selectors = SelectorList::parse_with_state(
+        parser,
+        input,
+        state |
+            SelectorParsingState::SKIP_DEFAULT_NAMESPACE |
+            SelectorParsingState::DISALLOW_PSEUDOS,
+        ParseErrorRecovery::DiscardList,
+    )?;
+    Ok(Component::NthOf(NthOfSelectorData::new(
+        &nth_data, selectors,
+    )))
+}
+
+/// Returns whether the name corresponds to a CSS2 pseudo-element that
+/// can be specified with the single colon syntax (in addition to the
+/// double-colon syntax, which can be used for all pseudo-elements).
+fn is_css2_pseudo_element(name: &str) -> bool {
+    // ** Do not add to this list! **
+    match_ignore_ascii_case! { name,
+        "before" | "after" | "first-line" | "first-letter" => true,
+        _ => false,
+    }
+}
+
+/// Parse a simple selector other than a type selector.
+///
+/// * `Err(())`: Invalid selector, abort
+/// * `Ok(None)`: Not a simple selector, could be something else. `input` was not consumed.
+/// * `Ok(Some(_))`: Parsed a simple selector or pseudo-element
+fn parse_one_simple_selector<'i, 't, P, Impl>(
+    parser: &P,
+    input: &mut CssParser<'i, 't>,
+    state: SelectorParsingState,
+) -> Result<Option<SimpleSelectorParseResult<Impl>>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    let start = input.state();
+    let token = match input.next_including_whitespace().map(|t| t.clone()) {
+        Ok(t) => t,
+        Err(..) => {
+            input.reset(&start);
+            return Ok(None);
+        },
+    };
+
+    Ok(Some(match token {
+        Token::IDHash(id) => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            let id = Component::ID(id.as_ref().into());
+            SimpleSelectorParseResult::SimpleSelector(id)
+        },
+        Token::Delim('.') => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            let location = input.current_source_location();
+            let class = match *input.next_including_whitespace()? {
+                Token::Ident(ref class) => class,
+                ref t => {
+                    let e = SelectorParseErrorKind::ClassNeedsIdent(t.clone());
+                    return Err(location.new_custom_error(e));
+                },
+            };
+            let class = Component::Class(class.as_ref().into());
+            SimpleSelectorParseResult::SimpleSelector(class)
+        },
+        Token::SquareBracketBlock => {
+            if state.intersects(SelectorParsingState::AFTER_PSEUDO) {
+                return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+            }
+            let attr = input.parse_nested_block(|input| parse_attribute_selector(parser, input))?;
+            SimpleSelectorParseResult::SimpleSelector(attr)
+        },
+        Token::Colon => {
+            let location = input.current_source_location();
+            let (is_single_colon, next_token) = match input.next_including_whitespace()?.clone() {
+                Token::Colon => (false, input.next_including_whitespace()?.clone()),
+                t => (true, t),
+            };
+            let (name, is_functional) = match next_token {
+                Token::Ident(name) => (name, false),
+                Token::Function(name) => (name, true),
+                t => {
+                    let e = SelectorParseErrorKind::PseudoElementExpectedIdent(t);
+                    return Err(input.new_custom_error(e));
+                },
+            };
+            let is_pseudo_element = !is_single_colon || is_css2_pseudo_element(&name);
+            if is_pseudo_element {
+                if !state.allows_pseudos() {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                let pseudo_element = if is_functional {
+                    if P::parse_part(parser) && name.eq_ignore_ascii_case("part") {
+                        if !state.allows_part() {
+                            return Err(
+                                input.new_custom_error(SelectorParseErrorKind::InvalidState)
+                            );
+                        }
+                        let names = input.parse_nested_block(|input| {
+                            let mut result = Vec::with_capacity(1);
+                            result.push(input.expect_ident()?.as_ref().into());
+                            while !input.is_exhausted() {
+                                result.push(input.expect_ident()?.as_ref().into());
+                            }
+                            Ok(result.into_boxed_slice())
+                        })?;
+                        return Ok(Some(SimpleSelectorParseResult::PartPseudo(names)));
+                    }
+                    if P::parse_slotted(parser) && name.eq_ignore_ascii_case("slotted") {
+                        if !state.allows_slotted() {
+                            return Err(
+                                input.new_custom_error(SelectorParseErrorKind::InvalidState)
+                            );
+                        }
+                        let selector = input.parse_nested_block(|input| {
+                            parse_inner_compound_selector(parser, input, state)
+                        })?;
+                        return Ok(Some(SimpleSelectorParseResult::SlottedPseudo(selector)));
+                    }
+                    input.parse_nested_block(|input| {
+                        P::parse_functional_pseudo_element(parser, name, input)
+                    })?
+                } else {
+                    P::parse_pseudo_element(parser, location, name)?
+                };
+
+                if state.intersects(SelectorParsingState::AFTER_SLOTTED) &&
+                    !pseudo_element.valid_after_slotted()
+                {
+                    return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
+                }
+                SimpleSelectorParseResult::PseudoElement(pseudo_element)
+            } else {
+                let pseudo_class = if is_functional {
+                    input.parse_nested_block(|input| {
+                        parse_functional_pseudo_class(parser, input, name, state)
+                    })?
+                } else {
+                    parse_simple_pseudo_class(parser, location, name, state)?
+                };
+                SimpleSelectorParseResult::SimpleSelector(pseudo_class)
+            }
+        },
+        _ => {
+            input.reset(&start);
+            return Ok(None);
+        },
+    }))
+}
+
+fn parse_simple_pseudo_class<'i, P, Impl>(
+    parser: &P,
+    location: SourceLocation,
+    name: CowRcStr<'i>,
+    state: SelectorParsingState,
+) -> Result<Component<Impl>, ParseError<'i, P::Error>>
+where
+    P: Parser<'i, Impl = Impl>,
+    Impl: SelectorImpl,
+{
+    if !state.allows_non_functional_pseudo_classes() {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+
+    if state.allows_tree_structural_pseudo_classes() {
+        match_ignore_ascii_case! { &name,
+            "first-child" => return Ok(Component::Nth(NthSelectorData::first(/* of_type = */ false))),
+            "last-child" => return Ok(Component::Nth(NthSelectorData::last(/* of_type = */ false))),
+            "only-child" => return Ok(Component::Nth(NthSelectorData::only(/* of_type = */ false))),
+            "root" => return Ok(Component::Root),
+            "empty" => return Ok(Component::Empty),
+            "scope" => return Ok(Component::Scope),
+            "host" if P::parse_host(parser) => return Ok(Component::Host(None)),
+            "first-of-type" => return Ok(Component::Nth(NthSelectorData::first(/* of_type = */ true))),
+            "last-of-type" => return Ok(Component::Nth(NthSelectorData::last(/* of_type = */ true))),
+            "only-of-type" => return Ok(Component::Nth(NthSelectorData::only(/* of_type = */ true))),
+            _ => {},
+        }
+    }
+
+    let pseudo_class = P::parse_non_ts_pseudo_class(parser, location, name)?;
+    if state.intersects(SelectorParsingState::AFTER_PSEUDO_ELEMENT) &&
+        !pseudo_class.is_user_action_state()
+    {
+        return Err(location.new_custom_error(SelectorParseErrorKind::InvalidState));
+    }
+    Ok(Component::NonTSPseudoClass(pseudo_class))
+}
+
+// NB: pub module in order to access the DummyParser
+#[cfg(test)]
+pub mod tests {
+    use super::*;
+    use crate::builder::SelectorFlags;
+    use crate::parser;
+    use cssparser::{serialize_identifier, Parser as CssParser, ParserInput, ToCss};
+    use std::collections::HashMap;
+    use std::fmt;
+
+    #[derive(Clone, Debug, Eq, PartialEq)]
+    pub enum PseudoClass {
+        Hover,
+        Active,
+        Lang(String),
+    }
+
+    #[derive(Clone, Debug, Eq, PartialEq)]
+    pub enum PseudoElement {
+        Before,
+        After,
+    }
+
+    impl parser::PseudoElement for PseudoElement {
+        type Impl = DummySelectorImpl;
+
+        fn accepts_state_pseudo_classes(&self) -> bool {
+            true
+        }
+
+        fn valid_after_slotted(&self) -> bool {
+            true
+        }
+    }
+
+    impl parser::NonTSPseudoClass for PseudoClass {
+        type Impl = DummySelectorImpl;
+
+        #[inline]
+        fn is_active_or_hover(&self) -> bool {
+            matches!(*self, PseudoClass::Active | PseudoClass::Hover)
+        }
+
+        #[inline]
+        fn is_user_action_state(&self) -> bool {
+            self.is_active_or_hover()
+        }
+    }
+
+    impl ToCss for PseudoClass {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where
+            W: fmt::Write,
+        {
+            match *self {
+                PseudoClass::Hover => dest.write_str(":hover"),
+                PseudoClass::Active => dest.write_str(":active"),
+                PseudoClass::Lang(ref lang) => {
+                    dest.write_str(":lang(")?;
+                    serialize_identifier(lang, dest)?;
+                    dest.write_char(')')
+                },
+            }
+        }
+    }
+
+    impl ToCss for PseudoElement {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where
+            W: fmt::Write,
+        {
+            match *self {
+                PseudoElement::Before => dest.write_str("::before"),
+                PseudoElement::After => dest.write_str("::after"),
+            }
+        }
+    }
+
+    #[derive(Clone, Debug, PartialEq)]
+    pub struct DummySelectorImpl;
+
+    #[derive(Default)]
+    pub struct DummyParser {
+        default_ns: Option<DummyAtom>,
+        ns_prefixes: HashMap<DummyAtom, DummyAtom>,
+    }
+
+    impl DummyParser {
+        fn default_with_namespace(default_ns: DummyAtom) -> DummyParser {
+            DummyParser {
+                default_ns: Some(default_ns),
+                ns_prefixes: Default::default(),
+            }
+        }
+    }
+
+    impl SelectorImpl for DummySelectorImpl {
+        type ExtraMatchingData<'a> = std::marker::PhantomData<&'a ()>;
+        type AttrValue = DummyAttrValue;
+        type Identifier = DummyAtom;
+        type LocalName = DummyAtom;
+        type NamespaceUrl = DummyAtom;
+        type NamespacePrefix = DummyAtom;
+        type BorrowedLocalName = DummyAtom;
+        type BorrowedNamespaceUrl = DummyAtom;
+        type NonTSPseudoClass = PseudoClass;
+        type PseudoElement = PseudoElement;
+    }
+
+    #[derive(Clone, Debug, Default, Eq, Hash, PartialEq)]
+    pub struct DummyAttrValue(String);
+
+    impl ToCss for DummyAttrValue {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where
+            W: fmt::Write,
+        {
+            use std::fmt::Write;
+
+            write!(cssparser::CssStringWriter::new(dest), "{}", &self.0)
+        }
+    }
+
+    impl<'a> From<&'a str> for DummyAttrValue {
+        fn from(string: &'a str) -> Self {
+            Self(string.into())
+        }
+    }
+
+    #[derive(Clone, Debug, Default, Eq, Hash, PartialEq)]
+    pub struct DummyAtom(String);
+
+    impl ToCss for DummyAtom {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where
+            W: fmt::Write,
+        {
+            serialize_identifier(&self.0, dest)
+        }
+    }
+
+    impl From<String> for DummyAtom {
+        fn from(string: String) -> Self {
+            DummyAtom(string)
+        }
+    }
+
+    impl<'a> From<&'a str> for DummyAtom {
+        fn from(string: &'a str) -> Self {
+            DummyAtom(string.into())
+        }
+    }
+
+    impl<'i> Parser<'i> for DummyParser {
+        type Impl = DummySelectorImpl;
+        type Error = SelectorParseErrorKind<'i>;
+
+        fn parse_slotted(&self) -> bool {
+            true
+        }
+
+        fn parse_nth_child_of(&self) -> bool {
+            true
+        }
+
+        fn parse_is_and_where(&self) -> bool {
+            true
+        }
+
+        fn parse_has(&self) -> bool {
+            true
+        }
+
+        fn parse_part(&self) -> bool {
+            true
+        }
+
+        fn parse_non_ts_pseudo_class(
+            &self,
+            location: SourceLocation,
+            name: CowRcStr<'i>,
+        ) -> Result<PseudoClass, SelectorParseError<'i>> {
+            match_ignore_ascii_case! { &name,
+                "hover" => return Ok(PseudoClass::Hover),
+                "active" => return Ok(PseudoClass::Active),
+                _ => {}
+            }
+            Err(
+                location.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                    name,
+                )),
+            )
+        }
+
+        fn parse_non_ts_functional_pseudo_class<'t>(
+            &self,
+            name: CowRcStr<'i>,
+            parser: &mut CssParser<'i, 't>,
+        ) -> Result<PseudoClass, SelectorParseError<'i>> {
+            match_ignore_ascii_case! { &name,
+                "lang" => {
+                    let lang = parser.expect_ident_or_string()?.as_ref().to_owned();
+                    return Ok(PseudoClass::Lang(lang));
+                },
+                _ => {}
+            }
+            Err(
+                parser.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                    name,
+                )),
+            )
+        }
+
+        fn parse_pseudo_element(
+            &self,
+            location: SourceLocation,
+            name: CowRcStr<'i>,
+        ) -> Result<PseudoElement, SelectorParseError<'i>> {
+            match_ignore_ascii_case! { &name,
+                "before" => return Ok(PseudoElement::Before),
+                "after" => return Ok(PseudoElement::After),
+                _ => {}
+            }
+            Err(
+                location.new_custom_error(SelectorParseErrorKind::UnsupportedPseudoClassOrElement(
+                    name,
+                )),
+            )
+        }
+
+        fn default_namespace(&self) -> Option<DummyAtom> {
+            self.default_ns.clone()
+        }
+
+        fn namespace_for_prefix(&self, prefix: &DummyAtom) -> Option<DummyAtom> {
+            self.ns_prefixes.get(prefix).cloned()
+        }
+    }
+
+    fn parse<'i>(
+        input: &'i str,
+    ) -> Result<SelectorList<DummySelectorImpl>, SelectorParseError<'i>> {
+        parse_ns(input, &DummyParser::default())
+    }
+
+    fn parse_expected<'i, 'a>(
+        input: &'i str,
+        expected: Option<&'a str>,
+    ) -> Result<SelectorList<DummySelectorImpl>, SelectorParseError<'i>> {
+        parse_ns_expected(input, &DummyParser::default(), expected)
+    }
+
+    fn parse_ns<'i>(
+        input: &'i str,
+        parser: &DummyParser,
+    ) -> Result<SelectorList<DummySelectorImpl>, SelectorParseError<'i>> {
+        parse_ns_expected(input, parser, None)
+    }
+
+    fn parse_ns_expected<'i, 'a>(
+        input: &'i str,
+        parser: &DummyParser,
+        expected: Option<&'a str>,
+    ) -> Result<SelectorList<DummySelectorImpl>, SelectorParseError<'i>> {
+        let mut parser_input = ParserInput::new(input);
+        let result = SelectorList::parse(parser, &mut CssParser::new(&mut parser_input));
+        if let Ok(ref selectors) = result {
+            assert_eq!(selectors.0.len(), 1);
+            // We can't assume that the serialized parsed selector will equal
+            // the input; for example, if there is no default namespace, '*|foo'
+            // should serialize to 'foo'.
+            assert_eq!(
+                selectors.0[0].to_css_string(),
+                match expected {
+                    Some(x) => x,
+                    None => input,
+                }
+            );
+        }
+        result
+    }
+
+    fn specificity(a: u32, b: u32, c: u32) -> u32 {
+        a << 20 | b << 10 | c
+    }
+
+    #[test]
+    fn test_empty() {
+        let mut input = ParserInput::new(":empty");
+        let list = SelectorList::parse(&DummyParser::default(), &mut CssParser::new(&mut input));
+        assert!(list.is_ok());
+    }
+
+    const MATHML: &str = "http://www.w3.org/1998/Math/MathML";
+    const SVG: &str = "http://www.w3.org/2000/svg";
+
+    #[test]
+    fn test_parsing() {
+        assert!(parse("").is_err());
+        assert!(parse(":lang(4)").is_err());
+        assert!(parse(":lang(en US)").is_err());
+        assert_eq!(
+            parse("EeÉ"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::LocalName(LocalName {
+                    name: DummyAtom::from("EeÉ"),
+                    lower_name: DummyAtom::from("eeÉ"),
+                })],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("|e"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ExplicitNoNamespace,
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("e"),
+                        lower_name: DummyAtom::from("e"),
+                    }),
+                ],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        // When the default namespace is not set, *| should be elided.
+        // https://github.com/servo/servo/pull/17537
+        assert_eq!(
+            parse_expected("*|e", Some("e")),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::LocalName(LocalName {
+                    name: DummyAtom::from("e"),
+                    lower_name: DummyAtom::from("e"),
+                })],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        // When the default namespace is set, *| should _not_ be elided (as foo
+        // is no longer equivalent to *|foo--the former is only for foo in the
+        // default namespace).
+        // https://github.com/servo/servo/issues/16020
+        assert_eq!(
+            parse_ns(
+                "*|e",
+                &DummyParser::default_with_namespace(DummyAtom::from("https://mozilla.org"))
+            ),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ExplicitAnyNamespace,
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("e"),
+                        lower_name: DummyAtom::from("e"),
+                    }),
+                ],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("*"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::ExplicitUniversalType],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("|*"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ExplicitNoNamespace,
+                    Component::ExplicitUniversalType,
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_expected("*|*", Some("*")),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::ExplicitUniversalType],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns(
+                "*|*",
+                &DummyParser::default_with_namespace(DummyAtom::from("https://mozilla.org"))
+            ),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ExplicitAnyNamespace,
+                    Component::ExplicitUniversalType,
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse(".foo:lang(en-US)"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Class(DummyAtom::from("foo")),
+                    Component::NonTSPseudoClass(PseudoClass::Lang("en-US".to_owned())),
+                ],
+                specificity(0, 2, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("#bar"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::ID(DummyAtom::from("bar"))],
+                specificity(1, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("e.foo#bar"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("e"),
+                        lower_name: DummyAtom::from("e"),
+                    }),
+                    Component::Class(DummyAtom::from("foo")),
+                    Component::ID(DummyAtom::from("bar")),
+                ],
+                specificity(1, 1, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("e.foo #bar"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("e"),
+                        lower_name: DummyAtom::from("e"),
+                    }),
+                    Component::Class(DummyAtom::from("foo")),
+                    Component::Combinator(Combinator::Descendant),
+                    Component::ID(DummyAtom::from("bar")),
+                ],
+                specificity(1, 1, 1),
+                Default::default(),
+            )]))
+        );
+        // Default namespace does not apply to attribute selectors
+        // https://github.com/mozilla/servo/pull/1652
+        let mut parser = DummyParser::default();
+        assert_eq!(
+            parse_ns("[Foo]", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::AttributeInNoNamespaceExists {
+                    local_name: DummyAtom::from("Foo"),
+                    local_name_lower: DummyAtom::from("foo"),
+                }],
+                specificity(0, 1, 0),
+                Default::default(),
+            )]))
+        );
+        assert!(parse_ns("svg|circle", &parser).is_err());
+        parser
+            .ns_prefixes
+            .insert(DummyAtom("svg".into()), DummyAtom(SVG.into()));
+        assert_eq!(
+            parse_ns("svg|circle", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Namespace(DummyAtom("svg".into()), SVG.into()),
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("circle"),
+                        lower_name: DummyAtom::from("circle"),
+                    }),
+                ],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns("svg|*", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Namespace(DummyAtom("svg".into()), SVG.into()),
+                    Component::ExplicitUniversalType,
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        // Default namespace does not apply to attribute selectors
+        // https://github.com/mozilla/servo/pull/1652
+        // but it does apply to implicit type selectors
+        // https://github.com/servo/rust-selectors/pull/82
+        parser.default_ns = Some(MATHML.into());
+        assert_eq!(
+            parse_ns("[Foo]", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::AttributeInNoNamespaceExists {
+                        local_name: DummyAtom::from("Foo"),
+                        local_name_lower: DummyAtom::from("foo"),
+                    },
+                ],
+                specificity(0, 1, 0),
+                Default::default(),
+            )]))
+        );
+        // Default namespace does apply to type selectors
+        assert_eq!(
+            parse_ns("e", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("e"),
+                        lower_name: DummyAtom::from("e"),
+                    }),
+                ],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns("*", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::ExplicitUniversalType,
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns("*|*", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ExplicitAnyNamespace,
+                    Component::ExplicitUniversalType,
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        // Default namespace applies to universal and type selectors inside :not and :matches,
+        // but not otherwise.
+        assert_eq!(
+            parse_ns(":not(.cl)", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::Negation(
+                        vec![Selector::from_vec(
+                            vec![Component::Class(DummyAtom::from("cl"))],
+                            specificity(0, 1, 0),
+                            Default::default(),
+                        )]
+                        .into_boxed_slice()
+                    ),
+                ],
+                specificity(0, 1, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns(":not(*)", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::Negation(
+                        vec![Selector::from_vec(
+                            vec![
+                                Component::DefaultNamespace(MATHML.into()),
+                                Component::ExplicitUniversalType,
+                            ],
+                            specificity(0, 0, 0),
+                            Default::default(),
+                        )]
+                        .into_boxed_slice(),
+                    ),
+                ],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns(":not(e)", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::DefaultNamespace(MATHML.into()),
+                    Component::Negation(
+                        vec![Selector::from_vec(
+                            vec![
+                                Component::DefaultNamespace(MATHML.into()),
+                                Component::LocalName(LocalName {
+                                    name: DummyAtom::from("e"),
+                                    lower_name: DummyAtom::from("e"),
+                                }),
+                            ],
+                            specificity(0, 0, 1),
+                            Default::default(),
+                        ),]
+                        .into_boxed_slice()
+                    ),
+                ],
+                specificity(0, 0, 1),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse("[attr|=\"foo\"]"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::AttributeInNoNamespace {
+                    local_name: DummyAtom::from("attr"),
+                    operator: AttrSelectorOperator::DashMatch,
+                    value: DummyAttrValue::from("foo"),
+                    never_matches: false,
+                    case_sensitivity: ParsedCaseSensitivity::CaseSensitive,
+                }],
+                specificity(0, 1, 0),
+                Default::default(),
+            )]))
+        );
+        // https://github.com/mozilla/servo/issues/1723
+        assert_eq!(
+            parse("::before"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Combinator(Combinator::PseudoElement),
+                    Component::PseudoElement(PseudoElement::Before),
+                ],
+                specificity(0, 0, 1),
+                SelectorFlags::HAS_PSEUDO,
+            )]))
+        );
+        assert_eq!(
+            parse("::before:hover"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Combinator(Combinator::PseudoElement),
+                    Component::PseudoElement(PseudoElement::Before),
+                    Component::NonTSPseudoClass(PseudoClass::Hover),
+                ],
+                specificity(0, 1, 1),
+                SelectorFlags::HAS_PSEUDO,
+            )]))
+        );
+        assert_eq!(
+            parse("::before:hover:hover"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Combinator(Combinator::PseudoElement),
+                    Component::PseudoElement(PseudoElement::Before),
+                    Component::NonTSPseudoClass(PseudoClass::Hover),
+                    Component::NonTSPseudoClass(PseudoClass::Hover),
+                ],
+                specificity(0, 2, 1),
+                SelectorFlags::HAS_PSEUDO,
+            )]))
+        );
+        assert!(parse("::before:hover:lang(foo)").is_err());
+        assert!(parse("::before:hover .foo").is_err());
+        assert!(parse("::before .foo").is_err());
+        assert!(parse("::before ~ bar").is_err());
+        assert!(parse("::before:active").is_ok());
+
+        // https://github.com/servo/servo/issues/15335
+        assert!(parse(":: before").is_err());
+        assert_eq!(
+            parse("div ::after"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::LocalName(LocalName {
+                        name: DummyAtom::from("div"),
+                        lower_name: DummyAtom::from("div"),
+                    }),
+                    Component::Combinator(Combinator::Descendant),
+                    Component::Combinator(Combinator::PseudoElement),
+                    Component::PseudoElement(PseudoElement::After),
+                ],
+                specificity(0, 0, 2),
+                SelectorFlags::HAS_PSEUDO,
+            )]))
+        );
+        assert_eq!(
+            parse("#d1 > .ok"),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ID(DummyAtom::from("d1")),
+                    Component::Combinator(Combinator::Child),
+                    Component::Class(DummyAtom::from("ok")),
+                ],
+                (1 << 20) + (1 << 10) + (0 << 0),
+                Default::default(),
+            )]))
+        );
+        parser.default_ns = None;
+        assert!(parse(":not(#provel.old)").is_ok());
+        assert!(parse(":not(#provel > old)").is_ok());
+        assert!(parse("table[rules]:not([rules=\"none\"]):not([rules=\"\"])").is_ok());
+        // https://github.com/servo/servo/issues/16017
+        assert_eq!(
+            parse_ns(":not(*)", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::Negation(
+                    vec![Selector::from_vec(
+                        vec![Component::ExplicitUniversalType],
+                        specificity(0, 0, 0),
+                        Default::default(),
+                    )]
+                    .into_boxed_slice()
+                )],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        assert_eq!(
+            parse_ns(":not(|*)", &parser),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::Negation(
+                    vec![Selector::from_vec(
+                        vec![
+                            Component::ExplicitNoNamespace,
+                            Component::ExplicitUniversalType,
+                        ],
+                        specificity(0, 0, 0),
+                        Default::default(),
+                    )]
+                    .into_boxed_slice(),
+                )],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+        // *| should be elided if there is no default namespace.
+        // https://github.com/servo/servo/pull/17537
+        assert_eq!(
+            parse_ns_expected(":not(*|*)", &parser, Some(":not(*)")),
+            Ok(SelectorList::from_vec(vec![Selector::from_vec(
+                vec![Component::Negation(
+                    vec![Selector::from_vec(
+                        vec![Component::ExplicitUniversalType],
+                        specificity(0, 0, 0),
+                        Default::default()
+                    )]
+                    .into_boxed_slice()
+                )],
+                specificity(0, 0, 0),
+                Default::default(),
+            )]))
+        );
+
+        assert!(parse("::slotted()").is_err());
+        assert!(parse("::slotted(div)").is_ok());
+        assert!(parse("::slotted(div).foo").is_err());
+        assert!(parse("::slotted(div + bar)").is_err());
+        assert!(parse("::slotted(div) + foo").is_err());
+
+        assert!(parse("::part()").is_err());
+        assert!(parse("::part(42)").is_err());
+        assert!(parse("::part(foo bar)").is_ok());
+        assert!(parse("::part(foo):hover").is_ok());
+        assert!(parse("::part(foo) + bar").is_err());
+
+        assert!(parse("div ::slotted(div)").is_ok());
+        assert!(parse("div + slot::slotted(div)").is_ok());
+        assert!(parse("div + slot::slotted(div.foo)").is_ok());
+        assert!(parse("slot::slotted(div,foo)::first-line").is_err());
+        assert!(parse("::slotted(div)::before").is_ok());
+        assert!(parse("slot::slotted(div,foo)").is_err());
+
+        assert!(parse("foo:where()").is_ok());
+        assert!(parse("foo:where(div, foo, .bar baz)").is_ok());
+        assert!(parse_expected("foo:where(::before)", Some("foo:where()")).is_ok());
+    }
+
+    #[test]
+    fn test_pseudo_iter() {
+        let selector = &parse("q::before").unwrap().0[0];
+        assert!(!selector.is_universal());
+        let mut iter = selector.iter();
+        assert_eq!(
+            iter.next(),
+            Some(&Component::PseudoElement(PseudoElement::Before))
+        );
+        assert_eq!(iter.next(), None);
+        let combinator = iter.next_sequence();
+        assert_eq!(combinator, Some(Combinator::PseudoElement));
+        assert!(matches!(iter.next(), Some(&Component::LocalName(..))));
+        assert_eq!(iter.next(), None);
+        assert_eq!(iter.next_sequence(), None);
+    }
+
+    #[test]
+    fn test_universal() {
+        let selector = &parse_ns(
+            "*|*::before",
+            &DummyParser::default_with_namespace(DummyAtom::from("https://mozilla.org")),
+        )
+        .unwrap()
+        .0[0];
+        assert!(selector.is_universal());
+    }
+
+    #[test]
+    fn test_empty_pseudo_iter() {
+        let selector = &parse("::before").unwrap().0[0];
+        assert!(selector.is_universal());
+        let mut iter = selector.iter();
+        assert_eq!(
+            iter.next(),
+            Some(&Component::PseudoElement(PseudoElement::Before))
+        );
+        assert_eq!(iter.next(), None);
+        assert_eq!(iter.next_sequence(), Some(Combinator::PseudoElement));
+        assert_eq!(iter.next(), None);
+        assert_eq!(iter.next_sequence(), None);
+    }
+
+    struct TestVisitor {
+        seen: Vec<String>,
+    }
+
+    impl SelectorVisitor for TestVisitor {
+        type Impl = DummySelectorImpl;
+
+        fn visit_simple_selector(&mut self, s: &Component<DummySelectorImpl>) -> bool {
+            let mut dest = String::new();
+            s.to_css(&mut dest).unwrap();
+            self.seen.push(dest);
+            true
+        }
+    }
+
+    #[test]
+    fn visitor() {
+        let mut test_visitor = TestVisitor { seen: vec![] };
+        parse(":not(:hover) ~ label").unwrap().0[0].visit(&mut test_visitor);
+        assert!(test_visitor.seen.contains(&":hover".into()));
+
+        let mut test_visitor = TestVisitor { seen: vec![] };
+        parse("::before:hover").unwrap().0[0].visit(&mut test_visitor);
+        assert!(test_visitor.seen.contains(&":hover".into()));
+    }
+}
diff --git a/servo/components/selectors/sink.rs b/servo/components/selectors/sink.rs
new file mode 100644
index 0000000000..dcdd7ff259
--- /dev/null
+++ b/servo/components/selectors/sink.rs
@@ -0,0 +1,31 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Small helpers to abstract over different containers.
+#![deny(missing_docs)]
+
+use smallvec::{Array, SmallVec};
+
+/// A trait to abstract over a `push` method that may be implemented for
+/// different kind of types.
+///
+/// Used to abstract over `Array`, `SmallVec` and `Vec`, and also to implement a
+/// type which `push` method does only tweak a byte when we only need to check
+/// for the presence of something.
+pub trait Push<T> {
+    /// Push a value into self.
+    fn push(&mut self, value: T);
+}
+
+impl<T> Push<T> for Vec<T> {
+    fn push(&mut self, value: T) {
+        Vec::push(self, value);
+    }
+}
+
+impl<A: Array> Push<A::Item> for SmallVec<A> {
+    fn push(&mut self, value: A::Item) {
+        SmallVec::push(self, value);
+    }
+}
diff --git a/servo/components/selectors/tree.rs b/servo/components/selectors/tree.rs
new file mode 100644
index 0000000000..7d6c1bed96
--- /dev/null
+++ b/servo/components/selectors/tree.rs
@@ -0,0 +1,171 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Traits that nodes must implement. Breaks the otherwise-cyclic dependency
+//! between layout and style.
+
+use crate::attr::{AttrSelectorOperation, CaseSensitivity, NamespaceConstraint};
+use crate::matching::{ElementSelectorFlags, MatchingContext};
+use crate::parser::SelectorImpl;
+use std::fmt::Debug;
+use std::ptr::NonNull;
+
+/// Opaque representation of an Element, for identity comparisons.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct OpaqueElement(NonNull<()>);
+
+unsafe impl Send for OpaqueElement {}
+
+impl OpaqueElement {
+    /// Creates a new OpaqueElement from an arbitrarily-typed pointer.
+    pub fn new<T>(ptr: &T) -> Self {
+        unsafe {
+            OpaqueElement(NonNull::new_unchecked(
+                ptr as *const T as *const () as *mut (),
+            ))
+        }
+    }
+}
+
+pub trait Element: Sized + Clone + Debug {
+    type Impl: SelectorImpl;
+
+    /// Converts self into an opaque representation.
+    fn opaque(&self) -> OpaqueElement;
+
+    fn parent_element(&self) -> Option<Self>;
+
+    /// Whether the parent node of this element is a shadow root.
+    fn parent_node_is_shadow_root(&self) -> bool;
+
+    /// The host of the containing shadow root, if any.
+    fn containing_shadow_host(&self) -> Option<Self>;
+
+    /// The parent of a given pseudo-element, after matching a pseudo-element
+    /// selector.
+    ///
+    /// This is guaranteed to be called in a pseudo-element.
+    fn pseudo_element_originating_element(&self) -> Option<Self> {
+        debug_assert!(self.is_pseudo_element());
+        self.parent_element()
+    }
+
+    /// Whether we're matching on a pseudo-element.
+    fn is_pseudo_element(&self) -> bool;
+
+    /// Skips non-element nodes
+    fn prev_sibling_element(&self) -> Option<Self>;
+
+    /// Skips non-element nodes
+    fn next_sibling_element(&self) -> Option<Self>;
+
+    /// Skips non-element nodes
+    fn first_element_child(&self) -> Option<Self>;
+
+    fn is_html_element_in_html_document(&self) -> bool;
+
+    fn has_local_name(&self, local_name: &<Self::Impl as SelectorImpl>::BorrowedLocalName) -> bool;
+
+    /// Empty string for no namespace
+    fn has_namespace(&self, ns: &<Self::Impl as SelectorImpl>::BorrowedNamespaceUrl) -> bool;
+
+    /// Whether this element and the `other` element have the same local name and namespace.
+    fn is_same_type(&self, other: &Self) -> bool;
+
+    fn attr_matches(
+        &self,
+        ns: &NamespaceConstraint<&<Self::Impl as SelectorImpl>::NamespaceUrl>,
+        local_name: &<Self::Impl as SelectorImpl>::LocalName,
+        operation: &AttrSelectorOperation<&<Self::Impl as SelectorImpl>::AttrValue>,
+    ) -> bool;
+
+    fn match_non_ts_pseudo_class(
+        &self,
+        pc: &<Self::Impl as SelectorImpl>::NonTSPseudoClass,
+        context: &mut MatchingContext<Self::Impl>,
+    ) -> bool;
+
+    fn match_pseudo_element(
+        &self,
+        pe: &<Self::Impl as SelectorImpl>::PseudoElement,
+        context: &mut MatchingContext<Self::Impl>,
+    ) -> bool;
+
+    /// Sets selector flags, which indicate what kinds of selectors may have
+    /// matched on this element and therefore what kind of work may need to
+    /// be performed when DOM state changes.
+    ///
+    /// You probably don't want to use this directly and want to use
+    /// apply_selector_flags, since that sets flags on the parent as needed.
+    fn set_selector_flags(&self, flags: ElementSelectorFlags);
+
+    fn apply_selector_flags(&self, flags: ElementSelectorFlags) {
+        // Handle flags that apply to the element.
+        let self_flags = flags.for_self();
+        if !self_flags.is_empty() {
+            self.set_selector_flags(self_flags);
+        }
+
+        // Handle flags that apply to the parent.
+        let parent_flags = flags.for_parent();
+        if !parent_flags.is_empty() {
+            if let Some(p) = self.parent_element() {
+                p.set_selector_flags(parent_flags);
+            }
+        }
+    }
+
+    /// Whether this element is a `link`.
+    fn is_link(&self) -> bool;
+
+    /// Returns whether the element is an HTML <slot> element.
+    fn is_html_slot_element(&self) -> bool;
+
+    /// Returns the assigned <slot> element this element is assigned to.
+    ///
+    /// Necessary for the `::slotted` pseudo-class.
+    fn assigned_slot(&self) -> Option<Self> {
+        None
+    }
+
+    fn has_id(
+        &self,
+        id: &<Self::Impl as SelectorImpl>::Identifier,
+        case_sensitivity: CaseSensitivity,
+    ) -> bool;
+
+    fn has_class(
+        &self,
+        name: &<Self::Impl as SelectorImpl>::Identifier,
+        case_sensitivity: CaseSensitivity,
+    ) -> bool;
+
+    /// Returns the mapping from the `exportparts` attribute in the reverse
+    /// direction, that is, in an outer-tree -> inner-tree direction.
+    fn imported_part(
+        &self,
+        name: &<Self::Impl as SelectorImpl>::Identifier,
+    ) -> Option<<Self::Impl as SelectorImpl>::Identifier>;
+
+    fn is_part(&self, name: &<Self::Impl as SelectorImpl>::Identifier) -> bool;
+
+    /// Returns whether this element matches `:empty`.
+    ///
+    /// That is, whether it does not contain any child element or any non-zero-length text node.
+    /// See http://dev.w3.org/csswg/selectors-3/#empty-pseudo
+    fn is_empty(&self) -> bool;
+
+    /// Returns whether this element matches `:root`,
+    /// i.e. whether it is the root element of a document.
+    ///
+    /// Note: this can be false even if `.parent_element()` is `None`
+    /// if the parent node is a `DocumentFragment`.
+    fn is_root(&self) -> bool;
+
+    /// Returns whether this element should ignore matching nth child
+    /// selector.
+    fn ignores_nth_child_selectors(&self) -> bool {
+        false
+    }
+}
diff --git a/servo/components/selectors/visitor.rs b/servo/components/selectors/visitor.rs
new file mode 100644
index 0000000000..3c0db6bb84
--- /dev/null
+++ b/servo/components/selectors/visitor.rs
@@ -0,0 +1,57 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Visitor traits for selectors.
+
+#![deny(missing_docs)]
+
+use crate::attr::NamespaceConstraint;
+use crate::parser::{Combinator, Component, Selector, SelectorImpl};
+
+/// A trait to visit selector properties.
+///
+/// All the `visit_foo` methods return a boolean indicating whether the
+/// traversal should continue or not.
+pub trait SelectorVisitor: Sized {
+    /// The selector implementation this visitor wants to visit.
+    type Impl: SelectorImpl;
+
+    /// Visit an attribute selector that may match (there are other selectors
+    /// that may never match, like those containing whitespace or the empty
+    /// string).
+    fn visit_attribute_selector(
+        &mut self,
+        _namespace: &NamespaceConstraint<&<Self::Impl as SelectorImpl>::NamespaceUrl>,
+        _local_name: &<Self::Impl as SelectorImpl>::LocalName,
+        _local_name_lower: &<Self::Impl as SelectorImpl>::LocalName,
+    ) -> bool {
+        true
+    }
+
+    /// Visit a simple selector.
+    fn visit_simple_selector(&mut self, _: &Component<Self::Impl>) -> bool {
+        true
+    }
+
+    /// Visit a nested selector list. The caller is responsible to call visit
+    /// into the internal selectors if / as needed.
+    ///
+    /// The default implementation does this.
+    fn visit_selector_list(&mut self, list: &[Selector<Self::Impl>]) -> bool {
+        for nested in list {
+            if !nested.visit(self) {
+                return false;
+            }
+        }
+        true
+    }
+
+    /// Visits a complex selector.
+    ///
+    /// Gets the combinator to the right of the selector, or `None` if the
+    /// selector is the rightmost one.
+    fn visit_complex_selector(&mut self, _combinator_to_right: Option<Combinator>) -> bool {
+        true
+    }
+}