Adding upstream version 124.0.1.upstream/124.0.1

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

6 files changed, 1774 insertions, 0 deletions

diff --git a/servo/components/style/rule_tree/core.rs b/servo/components/style/rule_tree/core.rs
new file mode 100644
index 0000000000..85584a0e22
--- /dev/null
+++ b/servo/components/style/rule_tree/core.rs
@@ -0,0 +1,772 @@
+/* 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/. */
+
+#![allow(unsafe_code)]
+
+use crate::applicable_declarations::CascadePriority;
+use crate::shared_lock::StylesheetGuards;
+use crate::stylesheets::layer_rule::LayerOrder;
+use malloc_size_of::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
+use parking_lot::RwLock;
+use smallvec::SmallVec;
+use std::fmt;
+use std::hash;
+use std::io::Write;
+use std::mem;
+use std::ptr;
+use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+
+use super::map::{Entry, Map};
+use super::unsafe_box::UnsafeBox;
+use super::{CascadeLevel, StyleSource};
+
+/// The rule tree, the structure servo uses to preserve the results of selector
+/// matching.
+///
+/// This is organized as a tree of rules. When a node matches a set of rules,
+/// they're inserted in order in the tree, starting with the less specific one.
+///
+/// When a rule is inserted in the tree, other elements may share the path up to
+/// a given rule. If that's the case, we don't duplicate child nodes, but share
+/// them.
+///
+/// When the rule node refcount drops to zero, it doesn't get freed. It gets
+/// instead put into a free list, and it is potentially GC'd after a while.
+///
+/// That way, a rule node that represents a likely-to-match-again rule (like a
+/// :hover rule) can be reused if we haven't GC'd it yet.
+#[derive(Debug)]
+pub struct RuleTree {
+    root: StrongRuleNode,
+}
+
+impl Drop for RuleTree {
+    fn drop(&mut self) {
+        unsafe { self.swap_free_list_and_gc(ptr::null_mut()) }
+    }
+}
+
+impl MallocSizeOf for RuleTree {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        let mut n = 0;
+        let mut stack = SmallVec::<[_; 32]>::new();
+        stack.push(self.root.clone());
+
+        while let Some(node) = stack.pop() {
+            n += unsafe { ops.malloc_size_of(&*node.p) };
+            let children = node.p.children.read();
+            children.shallow_size_of(ops);
+            for c in &*children {
+                stack.push(unsafe { c.upgrade() });
+            }
+        }
+
+        n
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+struct ChildKey(CascadePriority, ptr::NonNull<()>);
+unsafe impl Send for ChildKey {}
+unsafe impl Sync for ChildKey {}
+
+impl RuleTree {
+    /// Construct a new rule tree.
+    pub fn new() -> Self {
+        RuleTree {
+            root: StrongRuleNode::new(Box::new(RuleNode::root())),
+        }
+    }
+
+    /// Get the root rule node.
+    pub fn root(&self) -> &StrongRuleNode {
+        &self.root
+    }
+
+    /// This can only be called when no other threads is accessing this tree.
+    pub fn gc(&self) {
+        unsafe { self.swap_free_list_and_gc(RuleNode::DANGLING_PTR) }
+    }
+
+    /// This can only be called when no other threads is accessing this tree.
+    pub fn maybe_gc(&self) {
+        #[cfg(debug_assertions)]
+        self.maybe_dump_stats();
+
+        if self.root.p.approximate_free_count.load(Ordering::Relaxed) > RULE_TREE_GC_INTERVAL {
+            self.gc();
+        }
+    }
+
+    #[cfg(debug_assertions)]
+    fn maybe_dump_stats(&self) {
+        use itertools::Itertools;
+        use std::cell::Cell;
+        use std::time::{Duration, Instant};
+
+        if !log_enabled!(log::Level::Trace) {
+            return;
+        }
+
+        const RULE_TREE_STATS_INTERVAL: Duration = Duration::from_secs(2);
+
+        thread_local! {
+            pub static LAST_STATS: Cell<Instant> = Cell::new(Instant::now());
+        };
+
+        let should_dump = LAST_STATS.with(|s| {
+            let now = Instant::now();
+            if now.duration_since(s.get()) < RULE_TREE_STATS_INTERVAL {
+                return false;
+            }
+            s.set(now);
+            true
+        });
+
+        if !should_dump {
+            return;
+        }
+
+        let mut children_count = fxhash::FxHashMap::default();
+
+        let mut stack = SmallVec::<[_; 32]>::new();
+        stack.push(self.root.clone());
+        while let Some(node) = stack.pop() {
+            let children = node.p.children.read();
+            *children_count.entry(children.len()).or_insert(0) += 1;
+            for c in &*children {
+                stack.push(unsafe { c.upgrade() });
+            }
+        }
+
+        trace!("Rule tree stats:");
+        let counts = children_count.keys().sorted();
+        for count in counts {
+            trace!(" {} - {}", count, children_count[count]);
+        }
+    }
+
+    /// Steals the free list and drops its contents.
+    unsafe fn swap_free_list_and_gc(&self, ptr: *mut RuleNode) {
+        let root = &self.root.p;
+
+        debug_assert!(!root.next_free.load(Ordering::Relaxed).is_null());
+
+        // Reset the approximate free count to zero, as we are going to steal
+        // the free list.
+        root.approximate_free_count.store(0, Ordering::Relaxed);
+
+        // Steal the free list head. Memory loads on nodes while iterating it
+        // must observe any prior changes that occured so this requires
+        // acquire ordering, but there are no writes that need to be kept
+        // before this swap so there is no need for release.
+        let mut head = root.next_free.swap(ptr, Ordering::Acquire);
+
+        while head != RuleNode::DANGLING_PTR {
+            debug_assert!(!head.is_null());
+
+            let mut node = UnsafeBox::from_raw(head);
+
+            // The root node cannot go on the free list.
+            debug_assert!(node.root.is_some());
+
+            // The refcount of nodes on the free list never goes below 1.
+            debug_assert!(node.refcount.load(Ordering::Relaxed) > 0);
+
+            // No one else is currently writing to that field. Get the address
+            // of the next node in the free list and replace it with null,
+            // other threads will now consider that this node is not on the
+            // free list.
+            head = node.next_free.swap(ptr::null_mut(), Ordering::Relaxed);
+
+            // This release write synchronises with the acquire fence in
+            // `WeakRuleNode::upgrade`, making sure that if `upgrade` observes
+            // decrements the refcount to 0, it will also observe the
+            // `node.next_free` swap to null above.
+            if node.refcount.fetch_sub(1, Ordering::Release) == 1 {
+                // And given it observed the null swap above, it will need
+                // `pretend_to_be_on_free_list` to finish its job, writing
+                // `RuleNode::DANGLING_PTR` in `node.next_free`.
+                RuleNode::pretend_to_be_on_free_list(&node);
+
+                // Drop this node now that we just observed its refcount going
+                // down to zero.
+                RuleNode::drop_without_free_list(&mut node);
+            }
+        }
+    }
+}
+
+/// The number of RuleNodes added to the free list before we will consider
+/// doing a GC when calling maybe_gc().  (The value is copied from Gecko,
+/// where it likely did not result from a rigorous performance analysis.)
+const RULE_TREE_GC_INTERVAL: usize = 300;
+
+/// A node in the rule tree.
+struct RuleNode {
+    /// The root node. Only the root has no root pointer, for obvious reasons.
+    root: Option<WeakRuleNode>,
+
+    /// The parent rule node. Only the root has no parent.
+    parent: Option<StrongRuleNode>,
+
+    /// The actual style source, either coming from a selector in a StyleRule,
+    /// or a raw property declaration block (like the style attribute).
+    ///
+    /// None for the root node.
+    source: Option<StyleSource>,
+
+    /// The cascade level + layer order this rule is positioned at.
+    cascade_priority: CascadePriority,
+
+    /// The refcount of this node.
+    ///
+    /// Starts at one. Incremented in `StrongRuleNode::clone` and
+    /// `WeakRuleNode::upgrade`. Decremented in `StrongRuleNode::drop`
+    /// and `RuleTree::swap_free_list_and_gc`.
+    ///
+    /// If a non-root node's refcount reaches zero, it is incremented back to at
+    /// least one in `RuleNode::pretend_to_be_on_free_list` until the caller who
+    /// observed it dropping to zero had a chance to try to remove it from its
+    /// parent's children list.
+    ///
+    /// The refcount should never be decremented to zero if the value in
+    /// `next_free` is not null.
+    refcount: AtomicUsize,
+
+    /// Only used for the root, stores the number of free rule nodes that are
+    /// around.
+    approximate_free_count: AtomicUsize,
+
+    /// The children of a given rule node. Children remove themselves from here
+    /// when they go away.
+    children: RwLock<Map<ChildKey, WeakRuleNode>>,
+
+    /// This field has two different meanings depending on whether this is the
+    /// root node or not.
+    ///
+    /// If it is the root, it represents the head of the free list. It may be
+    /// null, which means the free list is gone because the tree was dropped,
+    /// and it may be `RuleNode::DANGLING_PTR`, which means the free list is
+    /// empty.
+    ///
+    /// If it is not the root node, this field is either null if the node is
+    /// not on the free list, `RuleNode::DANGLING_PTR` if it is the last item
+    /// on the free list or the node is pretending to be on the free list, or
+    /// any valid non-null pointer representing the next item on the free list
+    /// after this one.
+    ///
+    /// See `RuleNode::push_on_free_list`, `swap_free_list_and_gc`, and
+    /// `WeakRuleNode::upgrade`.
+    ///
+    /// Two threads should never attempt to put the same node on the free list
+    /// both at the same time.
+    next_free: AtomicPtr<RuleNode>,
+}
+
+// On Gecko builds, hook into the leak checking machinery.
+#[cfg(feature = "gecko_refcount_logging")]
+mod gecko_leak_checking {
+    use super::RuleNode;
+    use std::mem::size_of;
+    use std::os::raw::{c_char, c_void};
+
+    extern "C" {
+        fn NS_LogCtor(aPtr: *mut c_void, aTypeName: *const c_char, aSize: u32);
+        fn NS_LogDtor(aPtr: *mut c_void, aTypeName: *const c_char, aSize: u32);
+    }
+    static NAME: &'static [u8] = b"RuleNode\0";
+
+    /// Logs the creation of a heap-allocated object to Gecko's leak-checking machinery.
+    pub(super) fn log_ctor(ptr: *const RuleNode) {
+        let s = NAME as *const [u8] as *const u8 as *const c_char;
+        unsafe {
+            NS_LogCtor(ptr as *mut c_void, s, size_of::<RuleNode>() as u32);
+        }
+    }
+
+    /// Logs the destruction of a heap-allocated object to Gecko's leak-checking machinery.
+    pub(super) fn log_dtor(ptr: *const RuleNode) {
+        let s = NAME as *const [u8] as *const u8 as *const c_char;
+        unsafe {
+            NS_LogDtor(ptr as *mut c_void, s, size_of::<RuleNode>() as u32);
+        }
+    }
+}
+
+#[inline(always)]
+fn log_new(_ptr: *const RuleNode) {
+    #[cfg(feature = "gecko_refcount_logging")]
+    gecko_leak_checking::log_ctor(_ptr);
+}
+
+#[inline(always)]
+fn log_drop(_ptr: *const RuleNode) {
+    #[cfg(feature = "gecko_refcount_logging")]
+    gecko_leak_checking::log_dtor(_ptr);
+}
+
+impl RuleNode {
+    const DANGLING_PTR: *mut Self = ptr::NonNull::dangling().as_ptr();
+
+    unsafe fn new(
+        root: WeakRuleNode,
+        parent: StrongRuleNode,
+        source: StyleSource,
+        cascade_priority: CascadePriority,
+    ) -> Self {
+        debug_assert!(root.p.parent.is_none());
+        RuleNode {
+            root: Some(root),
+            parent: Some(parent),
+            source: Some(source),
+            cascade_priority,
+            refcount: AtomicUsize::new(1),
+            children: Default::default(),
+            approximate_free_count: AtomicUsize::new(0),
+            next_free: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    fn root() -> Self {
+        RuleNode {
+            root: None,
+            parent: None,
+            source: None,
+            cascade_priority: CascadePriority::new(CascadeLevel::UANormal, LayerOrder::root()),
+            refcount: AtomicUsize::new(1),
+            approximate_free_count: AtomicUsize::new(0),
+            children: Default::default(),
+            next_free: AtomicPtr::new(RuleNode::DANGLING_PTR),
+        }
+    }
+
+    fn key(&self) -> ChildKey {
+        ChildKey(
+            self.cascade_priority,
+            self.source
+                .as_ref()
+                .expect("Called key() on the root node")
+                .key(),
+        )
+    }
+
+    /// Drops a node without ever putting it on the free list.
+    ///
+    /// Note that the node may not be dropped if we observe that its refcount
+    /// isn't zero anymore when we write-lock its parent's children map to
+    /// remove it.
+    ///
+    /// This loops over parents of dropped nodes if their own refcount reaches
+    /// zero to avoid recursion when dropping deep hierarchies of nodes.
+    ///
+    /// For non-root nodes, this should always be preceded by a call of
+    /// `RuleNode::pretend_to_be_on_free_list`.
+    unsafe fn drop_without_free_list(this: &mut UnsafeBox<Self>) {
+        // We clone the box and shadow the original one to be able to loop
+        // over its ancestors if they also need to be dropped.
+        let mut this = UnsafeBox::clone(this);
+        loop {
+            // If the node has a parent, we need to remove it from its parent's
+            // children list.
+            if let Some(parent) = this.parent.as_ref() {
+                debug_assert!(!this.next_free.load(Ordering::Relaxed).is_null());
+
+                // We lock the parent's children list, which means no other
+                // thread will have any more opportunity to resurrect the node
+                // anymore.
+                let mut children = parent.p.children.write();
+
+                this.next_free.store(ptr::null_mut(), Ordering::Relaxed);
+
+                // We decrement the counter to remove the "pretend to be
+                // on the free list" reference.
+                let old_refcount = this.refcount.fetch_sub(1, Ordering::Release);
+                debug_assert!(old_refcount != 0);
+                if old_refcount != 1 {
+                    // Other threads resurrected this node and those references
+                    // are still alive, we have nothing to do anymore.
+                    return;
+                }
+
+                // We finally remove the node from its parent's children list,
+                // there are now no other references to it and it cannot
+                // be resurrected anymore even after we unlock the list.
+                debug!(
+                    "Remove from child list: {:?}, parent: {:?}",
+                    this.as_mut_ptr(),
+                    this.parent.as_ref().map(|p| p.p.as_mut_ptr())
+                );
+                let weak = children.remove(&this.key(), |node| node.p.key()).unwrap();
+                assert_eq!(weak.p.as_mut_ptr(), this.as_mut_ptr());
+            } else {
+                debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
+                debug_assert_eq!(this.refcount.load(Ordering::Relaxed), 0);
+            }
+
+            // We are going to drop this node for good this time, as per the
+            // usual refcounting protocol we need an acquire fence here before
+            // we run the destructor.
+            //
+            // See https://github.com/rust-lang/rust/pull/41714#issuecomment-298996916
+            // for why it doesn't matter whether this is a load or a fence.
+            atomic::fence(Ordering::Acquire);
+
+            // Remove the parent reference from the child to avoid
+            // recursively dropping it and putting it on the free list.
+            let parent = UnsafeBox::deref_mut(&mut this).parent.take();
+
+            // We now drop the actual box and its contents, no one should
+            // access the current value in `this` anymore.
+            log_drop(&*this);
+            UnsafeBox::drop(&mut this);
+
+            if let Some(parent) = parent {
+                // We will attempt to drop the node's parent without the free
+                // list, so we clone the inner unsafe box and forget the
+                // original parent to avoid running its `StrongRuleNode`
+                // destructor which would attempt to use the free list if it
+                // still exists.
+                this = UnsafeBox::clone(&parent.p);
+                mem::forget(parent);
+                if this.refcount.fetch_sub(1, Ordering::Release) == 1 {
+                    debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
+                    if this.root.is_some() {
+                        RuleNode::pretend_to_be_on_free_list(&this);
+                    }
+                    // Parent also reached refcount zero, we loop to drop it.
+                    continue;
+                }
+            }
+
+            return;
+        }
+    }
+
+    /// Pushes this node on the tree's free list. Returns false if the free list
+    /// is gone. Should only be called after we decremented a node's refcount
+    /// to zero and pretended to be on the free list.
+    unsafe fn push_on_free_list(this: &UnsafeBox<Self>) -> bool {
+        let root = &this.root.as_ref().unwrap().p;
+
+        debug_assert!(this.refcount.load(Ordering::Relaxed) > 0);
+        debug_assert_eq!(this.next_free.load(Ordering::Relaxed), Self::DANGLING_PTR);
+
+        // Increment the approximate free count by one.
+        root.approximate_free_count.fetch_add(1, Ordering::Relaxed);
+
+        // If the compare-exchange operation fails in the loop, we will retry
+        // with the new head value, so this can be a relaxed load.
+        let mut head = root.next_free.load(Ordering::Relaxed);
+
+        while !head.is_null() {
+            // Two threads can never attempt to push the same node on the free
+            // list both at the same time, so whoever else pushed a node on the
+            // free list cannot have done so with this node.
+            debug_assert_ne!(head, this.as_mut_ptr());
+
+            // Store the current head of the free list in this node.
+            this.next_free.store(head, Ordering::Relaxed);
+
+            // Any thread acquiring the free list must observe the previous
+            // next_free changes that occured, hence the release ordering
+            // on success.
+            match root.next_free.compare_exchange_weak(
+                head,
+                this.as_mut_ptr(),
+                Ordering::Release,
+                Ordering::Relaxed,
+            ) {
+                Ok(_) => {
+                    // This node is now on the free list, caller should not use
+                    // the node anymore.
+                    return true;
+                },
+                Err(new_head) => head = new_head,
+            }
+        }
+
+        // Tree was dropped and free list has been destroyed. We did not push
+        // this node on the free list but we still pretend to be on the free
+        // list to be ready to call `drop_without_free_list`.
+        false
+    }
+
+    /// Makes the node pretend to be on the free list. This will increment the
+    /// refcount by 1 and store `Self::DANGLING_PTR` in `next_free`. This
+    /// method should only be called after caller decremented the refcount to
+    /// zero, with the null pointer stored in `next_free`.
+    unsafe fn pretend_to_be_on_free_list(this: &UnsafeBox<Self>) {
+        debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
+        this.refcount.fetch_add(1, Ordering::Relaxed);
+        this.next_free.store(Self::DANGLING_PTR, Ordering::Release);
+    }
+
+    fn as_mut_ptr(&self) -> *mut RuleNode {
+        self as *const RuleNode as *mut RuleNode
+    }
+}
+
+pub(crate) struct WeakRuleNode {
+    p: UnsafeBox<RuleNode>,
+}
+
+/// A strong reference to a rule node.
+pub struct StrongRuleNode {
+    p: UnsafeBox<RuleNode>,
+}
+
+#[cfg(feature = "servo")]
+malloc_size_of_is_0!(StrongRuleNode);
+
+impl StrongRuleNode {
+    fn new(n: Box<RuleNode>) -> Self {
+        debug_assert_eq!(n.parent.is_none(), !n.source.is_some());
+
+        log_new(&*n);
+
+        debug!("Creating rule node: {:p}", &*n);
+
+        Self {
+            p: UnsafeBox::from_box(n),
+        }
+    }
+
+    unsafe fn from_unsafe_box(p: UnsafeBox<RuleNode>) -> Self {
+        Self { p }
+    }
+
+    unsafe fn downgrade(&self) -> WeakRuleNode {
+        WeakRuleNode {
+            p: UnsafeBox::clone(&self.p),
+        }
+    }
+
+    /// Get the parent rule node of this rule node.
+    pub fn parent(&self) -> Option<&StrongRuleNode> {
+        self.p.parent.as_ref()
+    }
+
+    pub(super) fn ensure_child(
+        &self,
+        root: &StrongRuleNode,
+        source: StyleSource,
+        cascade_priority: CascadePriority,
+    ) -> StrongRuleNode {
+        use parking_lot::RwLockUpgradableReadGuard;
+
+        debug_assert!(
+            self.p.cascade_priority <= cascade_priority,
+            "Should be ordered (instead {:?} > {:?}), from {:?} and {:?}",
+            self.p.cascade_priority,
+            cascade_priority,
+            self.p.source,
+            source,
+        );
+
+        let key = ChildKey(cascade_priority, source.key());
+        let children = self.p.children.upgradable_read();
+        if let Some(child) = children.get(&key, |node| node.p.key()) {
+            // Sound to call because we read-locked the parent's children.
+            return unsafe { child.upgrade() };
+        }
+        let mut children = RwLockUpgradableReadGuard::upgrade(children);
+        match children.entry(key, |node| node.p.key()) {
+            Entry::Occupied(child) => {
+                // Sound to call because we write-locked the parent's children.
+                unsafe { child.upgrade() }
+            },
+            Entry::Vacant(entry) => unsafe {
+                let node = StrongRuleNode::new(Box::new(RuleNode::new(
+                    root.downgrade(),
+                    self.clone(),
+                    source,
+                    cascade_priority,
+                )));
+                // Sound to call because we still own a strong reference to
+                // this node, through the `node` variable itself that we are
+                // going to return to the caller.
+                entry.insert(node.downgrade());
+                node
+            },
+        }
+    }
+
+    /// Get the style source corresponding to this rule node. May return `None`
+    /// if it's the root node, which means that the node hasn't matched any
+    /// rules.
+    pub fn style_source(&self) -> Option<&StyleSource> {
+        self.p.source.as_ref()
+    }
+
+    /// The cascade priority.
+    #[inline]
+    pub fn cascade_priority(&self) -> CascadePriority {
+        self.p.cascade_priority
+    }
+
+    /// The cascade level.
+    #[inline]
+    pub fn cascade_level(&self) -> CascadeLevel {
+        self.cascade_priority().cascade_level()
+    }
+
+    /// The importance.
+    #[inline]
+    pub fn importance(&self) -> crate::properties::Importance {
+        self.cascade_level().importance()
+    }
+
+    /// Returns whether this node has any child, only intended for testing
+    /// purposes.
+    pub unsafe fn has_children_for_testing(&self) -> bool {
+        !self.p.children.read().is_empty()
+    }
+
+    pub(super) fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W, indent: usize) {
+        const INDENT_INCREMENT: usize = 4;
+
+        for _ in 0..indent {
+            let _ = write!(writer, " ");
+        }
+
+        let _ = writeln!(
+            writer,
+            " - {:p} (ref: {:?}, parent: {:?})",
+            &*self.p,
+            self.p.refcount.load(Ordering::Relaxed),
+            self.parent().map(|p| &*p.p as *const RuleNode)
+        );
+
+        for _ in 0..indent {
+            let _ = write!(writer, " ");
+        }
+
+        if let Some(source) = self.style_source() {
+            source.dump(self.cascade_level().guard(guards), writer);
+        } else {
+            if indent != 0 {
+                warn!("How has this happened?");
+            }
+            let _ = write!(writer, "(root)");
+        }
+
+        let _ = write!(writer, "\n");
+        for child in &*self.p.children.read() {
+            unsafe {
+                child
+                    .upgrade()
+                    .dump(guards, writer, indent + INDENT_INCREMENT);
+            }
+        }
+    }
+}
+
+impl Clone for StrongRuleNode {
+    fn clone(&self) -> Self {
+        debug!(
+            "{:p}: {:?}+",
+            &*self.p,
+            self.p.refcount.load(Ordering::Relaxed)
+        );
+        debug_assert!(self.p.refcount.load(Ordering::Relaxed) > 0);
+        self.p.refcount.fetch_add(1, Ordering::Relaxed);
+        unsafe { StrongRuleNode::from_unsafe_box(UnsafeBox::clone(&self.p)) }
+    }
+}
+
+impl Drop for StrongRuleNode {
+    #[cfg_attr(feature = "servo", allow(unused_mut))]
+    fn drop(&mut self) {
+        let node = &*self.p;
+        debug!("{:p}: {:?}-", node, node.refcount.load(Ordering::Relaxed));
+        debug!(
+            "Dropping node: {:p}, root: {:?}, parent: {:?}",
+            node,
+            node.root.as_ref().map(|r| &*r.p as *const RuleNode),
+            node.parent.as_ref().map(|p| &*p.p as *const RuleNode)
+        );
+
+        let should_drop = {
+            debug_assert!(node.refcount.load(Ordering::Relaxed) > 0);
+            node.refcount.fetch_sub(1, Ordering::Release) == 1
+        };
+
+        if !should_drop {
+            // The refcount didn't even drop zero yet, there is nothing for us
+            // to do anymore.
+            return;
+        }
+
+        unsafe {
+            if node.root.is_some() {
+                // This is a non-root node and we just observed the refcount
+                // dropping to zero, we need to pretend to be on the free list
+                // to unstuck any thread who tried to resurrect this node first
+                // through `WeakRuleNode::upgrade`.
+                RuleNode::pretend_to_be_on_free_list(&self.p);
+
+                // Attempt to push the node on the free list. This may fail
+                // if the free list is gone.
+                if RuleNode::push_on_free_list(&self.p) {
+                    return;
+                }
+            }
+
+            // Either this was the last reference of the root node, or the
+            // tree rule is gone and there is no free list anymore. Drop the
+            // node.
+            RuleNode::drop_without_free_list(&mut self.p);
+        }
+    }
+}
+
+impl WeakRuleNode {
+    /// Upgrades this weak node reference, returning a strong one.
+    ///
+    /// Must be called with items stored in a node's children list. The children
+    /// list must at least be read-locked when this is called.
+    unsafe fn upgrade(&self) -> StrongRuleNode {
+        debug!("Upgrading weak node: {:p}", &*self.p);
+
+        if self.p.refcount.fetch_add(1, Ordering::Relaxed) == 0 {
+            // We observed a refcount of 0, we need to wait for this node to
+            // be put on the free list. Resetting the `next_free` pointer to
+            // null is only done in `RuleNode::drop_without_free_list`, just
+            // before a release refcount decrement, so this acquire fence here
+            // makes sure that we observed the write to null before we loop
+            // until there is a non-null value.
+            atomic::fence(Ordering::Acquire);
+            while self.p.next_free.load(Ordering::Relaxed).is_null() {}
+        }
+        StrongRuleNode::from_unsafe_box(UnsafeBox::clone(&self.p))
+    }
+}
+
+impl fmt::Debug for StrongRuleNode {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (&*self.p as *const RuleNode).fmt(f)
+    }
+}
+
+impl Eq for StrongRuleNode {}
+impl PartialEq for StrongRuleNode {
+    fn eq(&self, other: &Self) -> bool {
+        &*self.p as *const RuleNode == &*other.p
+    }
+}
+
+impl hash::Hash for StrongRuleNode {
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: hash::Hasher,
+    {
+        (&*self.p as *const RuleNode).hash(state)
+    }
+}
+
+// Large pages generate thousands of RuleNode objects.
+size_of_test!(RuleNode, 80);
+// StrongRuleNode should be pointer-sized even inside an option.
+size_of_test!(Option<StrongRuleNode>, 8);
diff --git a/servo/components/style/rule_tree/level.rs b/servo/components/style/rule_tree/level.rs
new file mode 100644
index 0000000000..b8cbe55ed9
--- /dev/null
+++ b/servo/components/style/rule_tree/level.rs
@@ -0,0 +1,249 @@
+/* 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/. */
+
+#![forbid(unsafe_code)]
+
+use crate::properties::Importance;
+use crate::shared_lock::{SharedRwLockReadGuard, StylesheetGuards};
+use crate::stylesheets::Origin;
+
+/// The cascade level these rules are relevant at, as per[1][2][3].
+///
+/// Presentational hints for SVG and HTML are in the "author-level
+/// zero-specificity" level, that is, right after user rules, and before author
+/// rules.
+///
+/// The order of variants declared here is significant, and must be in
+/// _ascending_ order of precedence.
+///
+/// See also [4] for the Shadow DOM bits. We rely on the invariant that rules
+/// from outside the tree the element is in can't affect the element.
+///
+/// The opposite is not true (i.e., :host and ::slotted) from an "inner" shadow
+/// tree may affect an element connected to the document or an "outer" shadow
+/// tree.
+///
+/// [1]: https://drafts.csswg.org/css-cascade/#cascade-origin
+/// [2]: https://drafts.csswg.org/css-cascade/#preshint
+/// [3]: https://html.spec.whatwg.org/multipage/#presentational-hints
+/// [4]: https://drafts.csswg.org/css-scoping/#shadow-cascading
+#[repr(u8)]
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, Ord, PartialEq, PartialOrd)]
+pub enum CascadeLevel {
+    /// Normal User-Agent rules.
+    UANormal,
+    /// User normal rules.
+    UserNormal,
+    /// Presentational hints.
+    PresHints,
+    /// Shadow DOM styles from author styles.
+    AuthorNormal {
+        /// The order in the shadow tree hierarchy. This number is relative to
+        /// the tree of the element, and thus the only invariants that need to
+        /// be preserved is:
+        ///
+        ///  * Zero is the same tree as the element that matched the rule. This
+        ///    is important so that we can optimize style attribute insertions.
+        ///
+        ///  * The levels are ordered in accordance with
+        ///    https://drafts.csswg.org/css-scoping/#shadow-cascading
+        shadow_cascade_order: ShadowCascadeOrder,
+    },
+    /// SVG SMIL animations.
+    SMILOverride,
+    /// CSS animations and script-generated animations.
+    Animations,
+    /// Author-supplied important rules.
+    AuthorImportant {
+        /// The order in the shadow tree hierarchy, inverted, so that PartialOrd
+        /// does the right thing.
+        shadow_cascade_order: ShadowCascadeOrder,
+    },
+    /// User important rules.
+    UserImportant,
+    /// User-agent important rules.
+    UAImportant,
+    /// Transitions
+    Transitions,
+}
+
+impl CascadeLevel {
+    /// Convert this level from "unimportant" to "important".
+    pub fn important(&self) -> Self {
+        match *self {
+            Self::UANormal => Self::UAImportant,
+            Self::UserNormal => Self::UserImportant,
+            Self::AuthorNormal {
+                shadow_cascade_order,
+            } => Self::AuthorImportant {
+                shadow_cascade_order: -shadow_cascade_order,
+            },
+            Self::PresHints |
+            Self::SMILOverride |
+            Self::Animations |
+            Self::AuthorImportant { .. } |
+            Self::UserImportant |
+            Self::UAImportant |
+            Self::Transitions => *self,
+        }
+    }
+
+    /// Convert this level from "important" to "non-important".
+    pub fn unimportant(&self) -> Self {
+        match *self {
+            Self::UAImportant => Self::UANormal,
+            Self::UserImportant => Self::UserNormal,
+            Self::AuthorImportant {
+                shadow_cascade_order,
+            } => Self::AuthorNormal {
+                shadow_cascade_order: -shadow_cascade_order,
+            },
+            Self::PresHints |
+            Self::SMILOverride |
+            Self::Animations |
+            Self::AuthorNormal { .. } |
+            Self::UserNormal |
+            Self::UANormal |
+            Self::Transitions => *self,
+        }
+    }
+
+    /// Select a lock guard for this level
+    pub fn guard<'a>(&self, guards: &'a StylesheetGuards<'a>) -> &'a SharedRwLockReadGuard<'a> {
+        match *self {
+            Self::UANormal | Self::UserNormal | Self::UserImportant | Self::UAImportant => {
+                guards.ua_or_user
+            },
+            _ => guards.author,
+        }
+    }
+
+    /// Returns the cascade level for author important declarations from the
+    /// same tree as the element.
+    #[inline]
+    pub fn same_tree_author_important() -> Self {
+        Self::AuthorImportant {
+            shadow_cascade_order: ShadowCascadeOrder::for_same_tree(),
+        }
+    }
+
+    /// Returns the cascade level for author normal declarations from the same
+    /// tree as the element.
+    #[inline]
+    pub fn same_tree_author_normal() -> Self {
+        Self::AuthorNormal {
+            shadow_cascade_order: ShadowCascadeOrder::for_same_tree(),
+        }
+    }
+
+    /// Returns whether this cascade level represents important rules of some
+    /// sort.
+    #[inline]
+    pub fn is_important(&self) -> bool {
+        match *self {
+            Self::AuthorImportant { .. } | Self::UserImportant | Self::UAImportant => true,
+            _ => false,
+        }
+    }
+
+    /// Returns the importance relevant for this rule. Pretty similar to
+    /// `is_important`.
+    #[inline]
+    pub fn importance(&self) -> Importance {
+        if self.is_important() {
+            Importance::Important
+        } else {
+            Importance::Normal
+        }
+    }
+
+    /// Returns the cascade origin of the rule.
+    #[inline]
+    pub fn origin(&self) -> Origin {
+        match *self {
+            Self::UAImportant | Self::UANormal => Origin::UserAgent,
+            Self::UserImportant | Self::UserNormal => Origin::User,
+            Self::PresHints |
+            Self::AuthorNormal { .. } |
+            Self::AuthorImportant { .. } |
+            Self::SMILOverride |
+            Self::Animations |
+            Self::Transitions => Origin::Author,
+        }
+    }
+
+    /// Returns whether this cascade level represents an animation rules.
+    #[inline]
+    pub fn is_animation(&self) -> bool {
+        match *self {
+            Self::SMILOverride | Self::Animations | Self::Transitions => true,
+            _ => false,
+        }
+    }
+}
+
+/// A counter to track how many shadow root rules deep we are. This is used to
+/// handle:
+///
+/// https://drafts.csswg.org/css-scoping/#shadow-cascading
+///
+/// See the static functions for the meaning of different values.
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, Ord, PartialEq, PartialOrd)]
+pub struct ShadowCascadeOrder(i8);
+
+impl ShadowCascadeOrder {
+    /// We keep a maximum of 3 bits of order as a limit so that we can pack
+    /// CascadeLevel in one byte by using half of it for the order, if that ends
+    /// up being necessary.
+    const MAX: i8 = 0b111;
+    const MIN: i8 = -Self::MAX;
+
+    /// A level for the outermost shadow tree (the shadow tree we own, and the
+    /// ones from the slots we're slotted in).
+    #[inline]
+    pub fn for_outermost_shadow_tree() -> Self {
+        Self(-1)
+    }
+
+    /// A level for the element's tree.
+    #[inline]
+    fn for_same_tree() -> Self {
+        Self(0)
+    }
+
+    /// A level for the innermost containing tree (the one closest to the
+    /// element).
+    #[inline]
+    pub fn for_innermost_containing_tree() -> Self {
+        Self(1)
+    }
+
+    /// Decrement the level, moving inwards. We should only move inwards if
+    /// we're traversing slots.
+    #[inline]
+    pub fn dec(&mut self) {
+        debug_assert!(self.0 < 0);
+        if self.0 != Self::MIN {
+            self.0 -= 1;
+        }
+    }
+
+    /// The level, moving inwards. We should only move inwards if we're
+    /// traversing slots.
+    #[inline]
+    pub fn inc(&mut self) {
+        debug_assert_ne!(self.0, -1);
+        if self.0 != Self::MAX {
+            self.0 += 1;
+        }
+    }
+}
+
+impl std::ops::Neg for ShadowCascadeOrder {
+    type Output = Self;
+    #[inline]
+    fn neg(self) -> Self {
+        Self(self.0.neg())
+    }
+}
diff --git a/servo/components/style/rule_tree/map.rs b/servo/components/style/rule_tree/map.rs
new file mode 100644
index 0000000000..33c470e9c1
--- /dev/null
+++ b/servo/components/style/rule_tree/map.rs
@@ -0,0 +1,201 @@
+/* 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/. */
+
+#![forbid(unsafe_code)]
+
+use fxhash::FxHashMap;
+use malloc_size_of::{MallocShallowSizeOf, MallocSizeOfOps};
+use std::collections::hash_map;
+use std::hash::Hash;
+use std::mem;
+
+pub(super) struct Map<K, V> {
+    inner: MapInner<K, V>,
+}
+
+enum MapInner<K, V> {
+    Empty,
+    One(V),
+    Map(Box<FxHashMap<K, V>>),
+}
+
+pub(super) struct MapIter<'a, K, V> {
+    inner: MapIterInner<'a, K, V>,
+}
+
+enum MapIterInner<'a, K, V> {
+    One(std::option::IntoIter<&'a V>),
+    Map(std::collections::hash_map::Values<'a, K, V>),
+}
+
+pub(super) enum Entry<'a, K, V> {
+    Occupied(&'a mut V),
+    Vacant(VacantEntry<'a, K, V>),
+}
+
+pub(super) struct VacantEntry<'a, K, V> {
+    inner: VacantEntryInner<'a, K, V>,
+}
+
+enum VacantEntryInner<'a, K, V> {
+    One(&'a mut MapInner<K, V>),
+    Map(hash_map::VacantEntry<'a, K, V>),
+}
+
+impl<K, V> Default for Map<K, V> {
+    fn default() -> Self {
+        Map {
+            inner: MapInner::Empty,
+        }
+    }
+}
+
+impl<'a, K, V> IntoIterator for &'a Map<K, V> {
+    type Item = &'a V;
+    type IntoIter = MapIter<'a, K, V>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        MapIter {
+            inner: match &self.inner {
+                MapInner::Empty => MapIterInner::One(None.into_iter()),
+                MapInner::One(one) => MapIterInner::One(Some(one).into_iter()),
+                MapInner::Map(map) => MapIterInner::Map(map.values()),
+            },
+        }
+    }
+}
+
+impl<'a, K, V> Iterator for MapIter<'a, K, V> {
+    type Item = &'a V;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match &mut self.inner {
+            MapIterInner::One(one_iter) => one_iter.next(),
+            MapIterInner::Map(map_iter) => map_iter.next(),
+        }
+    }
+}
+
+impl<K, V> Map<K, V>
+where
+    K: Eq + Hash,
+{
+    pub(super) fn is_empty(&self) -> bool {
+        match &self.inner {
+            MapInner::Empty => true,
+            MapInner::One(_) => false,
+            MapInner::Map(map) => map.is_empty(),
+        }
+    }
+
+    #[cfg(debug_assertions)]
+    pub(super) fn len(&self) -> usize {
+        match &self.inner {
+            MapInner::Empty => 0,
+            MapInner::One(_) => 1,
+            MapInner::Map(map) => map.len(),
+        }
+    }
+
+    pub(super) fn get(&self, key: &K, key_from_value: impl FnOnce(&V) -> K) -> Option<&V> {
+        match &self.inner {
+            MapInner::One(one) if *key == key_from_value(one) => Some(one),
+            MapInner::Map(map) => map.get(key),
+            MapInner::Empty | MapInner::One(_) => None,
+        }
+    }
+
+    pub(super) fn entry(
+        &mut self,
+        key: K,
+        key_from_value: impl FnOnce(&V) -> K,
+    ) -> Entry<'_, K, V> {
+        match self.inner {
+            ref mut inner @ MapInner::Empty => Entry::Vacant(VacantEntry {
+                inner: VacantEntryInner::One(inner),
+            }),
+            MapInner::One(_) => {
+                let one = match mem::replace(&mut self.inner, MapInner::Empty) {
+                    MapInner::One(one) => one,
+                    _ => unreachable!(),
+                };
+                // If this panics, the child `one` will be lost.
+                let one_key = key_from_value(&one);
+                // Same for the equality test.
+                if key == one_key {
+                    self.inner = MapInner::One(one);
+                    let one = match &mut self.inner {
+                        MapInner::One(one) => one,
+                        _ => unreachable!(),
+                    };
+                    return Entry::Occupied(one);
+                }
+                self.inner = MapInner::Map(Box::new(FxHashMap::with_capacity_and_hasher(
+                    2,
+                    Default::default(),
+                )));
+                let map = match &mut self.inner {
+                    MapInner::Map(map) => map,
+                    _ => unreachable!(),
+                };
+                map.insert(one_key, one);
+                match map.entry(key) {
+                    hash_map::Entry::Vacant(entry) => Entry::Vacant(VacantEntry {
+                        inner: VacantEntryInner::Map(entry),
+                    }),
+                    _ => unreachable!(),
+                }
+            },
+            MapInner::Map(ref mut map) => match map.entry(key) {
+                hash_map::Entry::Occupied(entry) => Entry::Occupied(entry.into_mut()),
+                hash_map::Entry::Vacant(entry) => Entry::Vacant(VacantEntry {
+                    inner: VacantEntryInner::Map(entry),
+                }),
+            },
+        }
+    }
+
+    pub(super) fn remove(&mut self, key: &K, key_from_value: impl FnOnce(&V) -> K) -> Option<V> {
+        match &mut self.inner {
+            MapInner::One(one) if *key == key_from_value(one) => {
+                match mem::replace(&mut self.inner, MapInner::Empty) {
+                    MapInner::One(one) => Some(one),
+                    _ => unreachable!(),
+                }
+            },
+            MapInner::Map(map) => map.remove(key),
+            MapInner::Empty | MapInner::One(_) => None,
+        }
+    }
+}
+
+impl<'a, K, V> VacantEntry<'a, K, V> {
+    pub(super) fn insert(self, value: V) -> &'a mut V {
+        match self.inner {
+            VacantEntryInner::One(map) => {
+                *map = MapInner::One(value);
+                match map {
+                    MapInner::One(one) => one,
+                    _ => unreachable!(),
+                }
+            },
+            VacantEntryInner::Map(entry) => entry.insert(value),
+        }
+    }
+}
+
+impl<K, V> MallocShallowSizeOf for Map<K, V>
+where
+    K: Eq + Hash,
+{
+    fn shallow_size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        match &self.inner {
+            MapInner::Map(m) => {
+                // We want to account for both the box and the hashmap.
+                m.shallow_size_of(ops) + (**m).shallow_size_of(ops)
+            },
+            MapInner::One(_) | MapInner::Empty => 0,
+        }
+    }
+}
diff --git a/servo/components/style/rule_tree/mod.rs b/servo/components/style/rule_tree/mod.rs
new file mode 100644
index 0000000000..18ee018d64
--- /dev/null
+++ b/servo/components/style/rule_tree/mod.rs
@@ -0,0 +1,403 @@
+/* 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/. */
+
+#![deny(unsafe_code)]
+
+//! The rule tree.
+
+use crate::applicable_declarations::{ApplicableDeclarationList, CascadePriority};
+use crate::properties::{LonghandIdSet, PropertyDeclarationBlock};
+use crate::shared_lock::{Locked, StylesheetGuards};
+use crate::stylesheets::layer_rule::LayerOrder;
+use servo_arc::ArcBorrow;
+use smallvec::SmallVec;
+use std::io::{self, Write};
+
+mod core;
+mod level;
+mod map;
+mod source;
+mod unsafe_box;
+
+pub use self::core::{RuleTree, StrongRuleNode};
+pub use self::level::{CascadeLevel, ShadowCascadeOrder};
+pub use self::source::StyleSource;
+
+impl RuleTree {
+    fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W) {
+        let _ = writeln!(writer, " + RuleTree");
+        self.root().dump(guards, writer, 0);
+    }
+
+    /// Dump the rule tree to stdout.
+    pub fn dump_stdout(&self, guards: &StylesheetGuards) {
+        let mut stdout = io::stdout();
+        self.dump(guards, &mut stdout);
+    }
+
+    /// Inserts the given rules, that must be in proper order by specifity, and
+    /// returns the corresponding rule node representing the last inserted one.
+    ///
+    /// !important rules are detected and inserted into the appropriate position
+    /// in the rule tree. This allows selector matching to ignore importance,
+    /// while still maintaining the appropriate cascade order in the rule tree.
+    pub fn insert_ordered_rules_with_important<'a, I>(
+        &self,
+        iter: I,
+        guards: &StylesheetGuards,
+    ) -> StrongRuleNode
+    where
+        I: Iterator<Item = (StyleSource, CascadePriority)>,
+    {
+        use self::CascadeLevel::*;
+        let mut current = self.root().clone();
+
+        let mut found_important = false;
+
+        let mut important_author = SmallVec::<[(StyleSource, CascadePriority); 4]>::new();
+        let mut important_user = SmallVec::<[(StyleSource, CascadePriority); 4]>::new();
+        let mut important_ua = SmallVec::<[(StyleSource, CascadePriority); 4]>::new();
+        let mut transition = None;
+
+        for (source, priority) in iter {
+            let level = priority.cascade_level();
+            debug_assert!(!level.is_important(), "Important levels handled internally");
+
+            let any_important = {
+                let pdb = source.read(level.guard(guards));
+                pdb.any_important()
+            };
+
+            if any_important {
+                found_important = true;
+                match level {
+                    AuthorNormal { .. } => {
+                        important_author.push((source.clone(), priority.important()))
+                    },
+                    UANormal => important_ua.push((source.clone(), priority.important())),
+                    UserNormal => important_user.push((source.clone(), priority.important())),
+                    _ => {},
+                };
+            }
+
+            // We don't optimize out empty rules, even though we could.
+            //
+            // Inspector relies on every rule being inserted in the normal level
+            // at least once, in order to return the rules with the correct
+            // specificity order.
+            //
+            // TODO(emilio): If we want to apply these optimizations without
+            // breaking inspector's expectations, we'd need to run
+            // selector-matching again at the inspector's request. That may or
+            // may not be a better trade-off.
+            if matches!(level, Transitions) && found_important {
+                // There can be at most one transition, and it will come at
+                // the end of the iterator. Stash it and apply it after
+                // !important rules.
+                debug_assert!(transition.is_none());
+                transition = Some(source);
+            } else {
+                current = current.ensure_child(self.root(), source, priority);
+            }
+        }
+
+        // Early-return in the common case of no !important declarations.
+        if !found_important {
+            return current;
+        }
+
+        // Insert important declarations, in order of increasing importance,
+        // followed by any transition rule.
+        //
+        // Important rules are sorted differently from unimportant ones by
+        // shadow order and cascade order.
+        if !important_author.is_empty() &&
+            important_author.first().unwrap().1 != important_author.last().unwrap().1
+        {
+            // We only need to sort if the important rules come from
+            // different trees, but we need this sort to be stable.
+            //
+            // FIXME(emilio): This could maybe be smarter, probably by chunking
+            // the important rules while inserting, and iterating the outer
+            // chunks in reverse order.
+            //
+            // That is, if we have rules with levels like: -1 -1 -1 0 0 0 1 1 1,
+            // we're really only sorting the chunks, while keeping elements
+            // inside the same chunk already sorted. Seems like we could try to
+            // keep a SmallVec-of-SmallVecs with the chunks and just iterate the
+            // outer in reverse.
+            important_author.sort_by_key(|&(_, priority)| priority);
+        }
+
+        for (source, priority) in important_author.drain(..) {
+            current = current.ensure_child(self.root(), source, priority);
+        }
+
+        for (source, priority) in important_user.drain(..) {
+            current = current.ensure_child(self.root(), source, priority);
+        }
+
+        for (source, priority) in important_ua.drain(..) {
+            current = current.ensure_child(self.root(), source, priority);
+        }
+
+        if let Some(source) = transition {
+            current = current.ensure_child(
+                self.root(),
+                source,
+                CascadePriority::new(Transitions, LayerOrder::root()),
+            );
+        }
+
+        current
+    }
+
+    /// Given a list of applicable declarations, insert the rules and return the
+    /// corresponding rule node.
+    pub fn compute_rule_node(
+        &self,
+        applicable_declarations: &mut ApplicableDeclarationList,
+        guards: &StylesheetGuards,
+    ) -> StrongRuleNode {
+        self.insert_ordered_rules_with_important(
+            applicable_declarations.drain(..).map(|d| d.for_rule_tree()),
+            guards,
+        )
+    }
+
+    /// Insert the given rules, that must be in proper order by specifity, and
+    /// return the corresponding rule node representing the last inserted one.
+    pub fn insert_ordered_rules<'a, I>(&self, iter: I) -> StrongRuleNode
+    where
+        I: Iterator<Item = (StyleSource, CascadePriority)>,
+    {
+        self.insert_ordered_rules_from(self.root().clone(), iter)
+    }
+
+    fn insert_ordered_rules_from<'a, I>(&self, from: StrongRuleNode, iter: I) -> StrongRuleNode
+    where
+        I: Iterator<Item = (StyleSource, CascadePriority)>,
+    {
+        let mut current = from;
+        for (source, priority) in iter {
+            current = current.ensure_child(self.root(), source, priority);
+        }
+        current
+    }
+
+    /// Replaces a rule in a given level (if present) for another rule.
+    ///
+    /// Returns the resulting node that represents the new path, or None if
+    /// the old path is still valid.
+    pub fn update_rule_at_level(
+        &self,
+        level: CascadeLevel,
+        layer_order: LayerOrder,
+        pdb: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
+        path: &StrongRuleNode,
+        guards: &StylesheetGuards,
+        important_rules_changed: &mut bool,
+    ) -> Option<StrongRuleNode> {
+        // TODO(emilio): Being smarter with lifetimes we could avoid a bit of
+        // the refcount churn.
+        let mut current = path.clone();
+        *important_rules_changed = false;
+
+        // First walk up until the first less-or-equally specific rule.
+        let mut children = SmallVec::<[_; 10]>::new();
+        while current.cascade_priority().cascade_level() > level {
+            children.push((
+                current.style_source().unwrap().clone(),
+                current.cascade_priority(),
+            ));
+            current = current.parent().unwrap().clone();
+        }
+
+        // Then remove the one at the level we want to replace, if any.
+        //
+        // NOTE: Here we assume that only one rule can be at the level we're
+        // replacing.
+        //
+        // This is certainly true for HTML style attribute rules, animations and
+        // transitions, but could not be so for SMIL animations, which we'd need
+        // to special-case (isn't hard, it's just about removing the `if` and
+        // special cases, and replacing them for a `while` loop, avoiding the
+        // optimizations).
+        if current.cascade_priority().cascade_level() == level {
+            *important_rules_changed |= level.is_important();
+
+            let current_decls = current.style_source().unwrap().as_declarations();
+
+            // If the only rule at the level we're replacing is exactly the
+            // same as `pdb`, we're done, and `path` is still valid.
+            if let (Some(ref pdb), Some(ref current_decls)) = (pdb, current_decls) {
+                // If the only rule at the level we're replacing is exactly the
+                // same as `pdb`, we're done, and `path` is still valid.
+                //
+                // TODO(emilio): Another potential optimization is the one where
+                // we can just replace the rule at that level for `pdb`, and
+                // then we don't need to re-create the children, and `path` is
+                // also equally valid. This is less likely, and would require an
+                // in-place mutation of the source, which is, at best, fiddly,
+                // so let's skip it for now.
+                let is_here_already = ArcBorrow::ptr_eq(pdb, current_decls);
+                if is_here_already {
+                    debug!("Picking the fast path in rule replacement");
+                    return None;
+                }
+            }
+
+            if current_decls.is_some() {
+                current = current.parent().unwrap().clone();
+            }
+        }
+
+        // Insert the rule if it's relevant at this level in the cascade.
+        //
+        // These optimizations are likely to be important, because the levels
+        // where replacements apply (style and animations) tend to trigger
+        // pretty bad styling cases already.
+        if let Some(pdb) = pdb {
+            if level.is_important() {
+                if pdb.read_with(level.guard(guards)).any_important() {
+                    current = current.ensure_child(
+                        self.root(),
+                        StyleSource::from_declarations(pdb.clone_arc()),
+                        CascadePriority::new(level, layer_order),
+                    );
+                    *important_rules_changed = true;
+                }
+            } else {
+                if pdb.read_with(level.guard(guards)).any_normal() {
+                    current = current.ensure_child(
+                        self.root(),
+                        StyleSource::from_declarations(pdb.clone_arc()),
+                        CascadePriority::new(level, layer_order),
+                    );
+                }
+            }
+        }
+
+        // Now the rule is in the relevant place, push the children as
+        // necessary.
+        let rule = self.insert_ordered_rules_from(current, children.drain(..).rev());
+        Some(rule)
+    }
+
+    /// Returns new rule nodes without Transitions level rule.
+    pub fn remove_transition_rule_if_applicable(&self, path: &StrongRuleNode) -> StrongRuleNode {
+        // Return a clone if there is no transition level.
+        if path.cascade_level() != CascadeLevel::Transitions {
+            return path.clone();
+        }
+
+        path.parent().unwrap().clone()
+    }
+
+    /// Returns new rule node without rules from declarative animations.
+    pub fn remove_animation_rules(&self, path: &StrongRuleNode) -> StrongRuleNode {
+        // Return a clone if there are no animation rules.
+        if !path.has_animation_or_transition_rules() {
+            return path.clone();
+        }
+
+        let iter = path
+            .self_and_ancestors()
+            .take_while(|node| node.cascade_level() >= CascadeLevel::SMILOverride);
+        let mut last = path;
+        let mut children = SmallVec::<[_; 10]>::new();
+        for node in iter {
+            if !node.cascade_level().is_animation() {
+                children.push((
+                    node.style_source().unwrap().clone(),
+                    node.cascade_priority(),
+                ));
+            }
+            last = node;
+        }
+
+        let rule = self
+            .insert_ordered_rules_from(last.parent().unwrap().clone(), children.drain(..).rev());
+        rule
+    }
+}
+
+impl StrongRuleNode {
+    /// Get an iterator for this rule node and its ancestors.
+    pub fn self_and_ancestors(&self) -> SelfAndAncestors {
+        SelfAndAncestors {
+            current: Some(self),
+        }
+    }
+
+    /// Returns true if there is either animation or transition level rule.
+    pub fn has_animation_or_transition_rules(&self) -> bool {
+        self.self_and_ancestors()
+            .take_while(|node| node.cascade_level() >= CascadeLevel::SMILOverride)
+            .any(|node| node.cascade_level().is_animation())
+    }
+
+    /// Get a set of properties whose CascadeLevel are higher than Animations
+    /// but not equal to Transitions.
+    ///
+    /// If there are any custom properties, we set the boolean value of the
+    /// returned tuple to true.
+    pub fn get_properties_overriding_animations(
+        &self,
+        guards: &StylesheetGuards,
+    ) -> (LonghandIdSet, bool) {
+        use crate::properties::PropertyDeclarationId;
+
+        // We want to iterate over cascade levels that override the animations
+        // level, i.e.  !important levels and the transitions level.
+        //
+        // However, we actually want to skip the transitions level because
+        // although it is higher in the cascade than animations, when both
+        // transitions and animations are present for a given element and
+        // property, transitions are suppressed so that they don't actually
+        // override animations.
+        let iter = self
+            .self_and_ancestors()
+            .skip_while(|node| node.cascade_level() == CascadeLevel::Transitions)
+            .take_while(|node| node.cascade_level() > CascadeLevel::Animations);
+        let mut result = (LonghandIdSet::new(), false);
+        for node in iter {
+            let style = node.style_source().unwrap();
+            for (decl, important) in style
+                .read(node.cascade_level().guard(guards))
+                .declaration_importance_iter()
+            {
+                // Although we are only iterating over cascade levels that
+                // override animations, in a given property declaration block we
+                // can have a mixture of !important and non-!important
+                // declarations but only the !important declarations actually
+                // override animations.
+                if important.important() {
+                    match decl.id() {
+                        PropertyDeclarationId::Longhand(id) => result.0.insert(id),
+                        PropertyDeclarationId::Custom(_) => result.1 = true,
+                    }
+                }
+            }
+        }
+        result
+    }
+}
+
+/// An iterator over a rule node and its ancestors.
+#[derive(Clone)]
+pub struct SelfAndAncestors<'a> {
+    current: Option<&'a StrongRuleNode>,
+}
+
+impl<'a> Iterator for SelfAndAncestors<'a> {
+    type Item = &'a StrongRuleNode;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.current.map(|node| {
+            self.current = node.parent();
+            node
+        })
+    }
+}
diff --git a/servo/components/style/rule_tree/source.rs b/servo/components/style/rule_tree/source.rs
new file mode 100644
index 0000000000..76443692d7
--- /dev/null
+++ b/servo/components/style/rule_tree/source.rs
@@ -0,0 +1,75 @@
+/* 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/. */
+
+#![forbid(unsafe_code)]
+
+use crate::properties::PropertyDeclarationBlock;
+use crate::shared_lock::{Locked, SharedRwLockReadGuard};
+use crate::stylesheets::StyleRule;
+use servo_arc::{Arc, ArcBorrow, ArcUnion, ArcUnionBorrow};
+use std::io::Write;
+use std::ptr;
+
+/// A style source for the rule node. It can either be a CSS style rule or a
+/// declaration block.
+///
+/// Note that, even though the declaration block from inside the style rule
+/// could be enough to implement the rule tree, keeping the whole rule provides
+/// more debuggability, and also the ability of show those selectors to
+/// devtools.
+#[derive(Clone, Debug)]
+pub struct StyleSource(ArcUnion<Locked<StyleRule>, Locked<PropertyDeclarationBlock>>);
+
+impl PartialEq for StyleSource {
+    fn eq(&self, other: &Self) -> bool {
+        ArcUnion::ptr_eq(&self.0, &other.0)
+    }
+}
+
+impl StyleSource {
+    /// Creates a StyleSource from a StyleRule.
+    pub fn from_rule(rule: Arc<Locked<StyleRule>>) -> Self {
+        StyleSource(ArcUnion::from_first(rule))
+    }
+
+    #[inline]
+    pub(super) fn key(&self) -> ptr::NonNull<()> {
+        self.0.ptr()
+    }
+
+    /// Creates a StyleSource from a PropertyDeclarationBlock.
+    pub fn from_declarations(decls: Arc<Locked<PropertyDeclarationBlock>>) -> Self {
+        StyleSource(ArcUnion::from_second(decls))
+    }
+
+    pub(super) fn dump<W: Write>(&self, guard: &SharedRwLockReadGuard, writer: &mut W) {
+        if let Some(ref rule) = self.0.as_first() {
+            let rule = rule.read_with(guard);
+            let _ = write!(writer, "{:?}", rule.selectors);
+        }
+
+        let _ = write!(writer, "  -> {:?}", self.read(guard).declarations());
+    }
+
+    /// Read the style source guard, and obtain thus read access to the
+    /// underlying property declaration block.
+    #[inline]
+    pub fn read<'a>(&'a self, guard: &'a SharedRwLockReadGuard) -> &'a PropertyDeclarationBlock {
+        let block: &Locked<PropertyDeclarationBlock> = match self.0.borrow() {
+            ArcUnionBorrow::First(ref rule) => &rule.get().read_with(guard).block,
+            ArcUnionBorrow::Second(ref block) => block.get(),
+        };
+        block.read_with(guard)
+    }
+
+    /// Returns the style rule if applicable, otherwise None.
+    pub fn as_rule(&self) -> Option<ArcBorrow<Locked<StyleRule>>> {
+        self.0.as_first()
+    }
+
+    /// Returns the declaration block if applicable, otherwise None.
+    pub fn as_declarations(&self) -> Option<ArcBorrow<Locked<PropertyDeclarationBlock>>> {
+        self.0.as_second()
+    }
+}
diff --git a/servo/components/style/rule_tree/unsafe_box.rs b/servo/components/style/rule_tree/unsafe_box.rs
new file mode 100644
index 0000000000..eaa441d7b2
--- /dev/null
+++ b/servo/components/style/rule_tree/unsafe_box.rs
@@ -0,0 +1,74 @@
+/* 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/. */
+
+#![allow(unsafe_code)]
+
+use std::mem::ManuallyDrop;
+use std::ops::Deref;
+use std::ptr;
+
+/// An unsafe box, derefs to `T`.
+pub(super) struct UnsafeBox<T> {
+    inner: ManuallyDrop<Box<T>>,
+}
+
+impl<T> UnsafeBox<T> {
+    /// Creates a new unsafe box.
+    pub(super) fn from_box(value: Box<T>) -> Self {
+        Self {
+            inner: ManuallyDrop::new(value),
+        }
+    }
+
+    /// Creates a new box from a pointer.
+    ///
+    /// # Safety
+    ///
+    /// The input should point to a valid `T`.
+    pub(super) unsafe fn from_raw(ptr: *mut T) -> Self {
+        Self {
+            inner: ManuallyDrop::new(Box::from_raw(ptr)),
+        }
+    }
+
+    /// Creates a new unsafe box from an existing one.
+    ///
+    /// # Safety
+    ///
+    /// There is no refcounting or whatever else in an unsafe box, so this
+    /// operation can lead to double frees.
+    pub(super) unsafe fn clone(this: &Self) -> Self {
+        Self {
+            inner: ptr::read(&this.inner),
+        }
+    }
+
+    /// Returns a mutable reference to the inner value of this unsafe box.
+    ///
+    /// # Safety
+    ///
+    /// Given `Self::clone`, nothing prevents anyone from creating
+    /// multiple mutable references to the inner value, which is completely UB.
+    pub(crate) unsafe fn deref_mut(this: &mut Self) -> &mut T {
+        &mut this.inner
+    }
+
+    /// Drops the inner value of this unsafe box.
+    ///
+    /// # Safety
+    ///
+    /// Given this doesn't consume the unsafe box itself, this has the same
+    /// safety caveats as `ManuallyDrop::drop`.
+    pub(super) unsafe fn drop(this: &mut Self) {
+        ManuallyDrop::drop(&mut this.inner)
+    }
+}
+
+impl<T> Deref for UnsafeBox<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}