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diff --git a/servo/components/to_shmem/lib.rs b/servo/components/to_shmem/lib.rs
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+/* 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/. */
+
+//! Trait for cloning data into a shared memory buffer.
+//!
+//! This module contains the SharedMemoryBuilder type and ToShmem trait.
+//!
+//! We put them here (and not in style_traits) so that we can derive ToShmem
+//! from the selectors and style crates.
+
+#![crate_name = "to_shmem"]
+#![crate_type = "rlib"]
+
+extern crate cssparser;
+extern crate servo_arc;
+extern crate smallbitvec;
+extern crate smallvec;
+#[cfg(feature = "string_cache")]
+extern crate string_cache;
+
+use servo_arc::{Arc, ThinArc};
+use smallbitvec::{InternalStorage, SmallBitVec};
+use smallvec::{Array, SmallVec};
+use std::alloc::Layout;
+#[cfg(debug_assertions)]
+use std::collections::HashSet;
+use std::ffi::CString;
+use std::isize;
+use std::marker::PhantomData;
+use std::mem::{self, ManuallyDrop};
+use std::num::Wrapping;
+use std::ops::Range;
+use std::os::raw::c_char;
+#[cfg(debug_assertions)]
+use std::os::raw::c_void;
+use std::ptr::{self, NonNull};
+use std::slice;
+use std::str;
+
+/// Result type for ToShmem::to_shmem.
+///
+/// The String is an error message describing why the call failed.
+pub type Result<T> = std::result::Result<ManuallyDrop<T>, String>;
+
+// Various pointer arithmetic functions in this file can be replaced with
+// functions on `Layout` once they have stabilized:
+//
+// https://github.com/rust-lang/rust/issues/55724
+
+/// A builder object that transforms and copies values into a fixed size buffer.
+pub struct SharedMemoryBuilder {
+    /// The buffer into which values will be copied.
+    buffer: *mut u8,
+    /// The size of the buffer.
+    capacity: usize,
+    /// The current position in the buffer, where the next value will be written
+    /// at.
+    index: usize,
+    /// Pointers to every shareable value that we store in the shared memory
+    /// buffer.  We use this to assert against encountering the same value
+    /// twice, e.g. through another Arc reference, so that we don't
+    /// inadvertently store duplicate copies of values.
+    #[cfg(debug_assertions)]
+    shared_values: HashSet<*const c_void>,
+}
+
+/// Amount of padding needed after `size` bytes to ensure that the following
+/// address will satisfy `align`.
+fn padding_needed_for(size: usize, align: usize) -> usize {
+    padded_size(size, align).wrapping_sub(size)
+}
+
+/// Rounds up `size` so that the following address will satisfy `align`.
+fn padded_size(size: usize, align: usize) -> usize {
+    size.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1)
+}
+
+impl SharedMemoryBuilder {
+    /// Creates a new SharedMemoryBuilder using the specified buffer.
+    pub unsafe fn new(buffer: *mut u8, capacity: usize) -> SharedMemoryBuilder {
+        SharedMemoryBuilder {
+            buffer,
+            capacity,
+            index: 0,
+            #[cfg(debug_assertions)]
+            shared_values: HashSet::new(),
+        }
+    }
+
+    /// Returns the number of bytes currently used in the buffer.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.index
+    }
+
+    /// Writes a value into the shared memory buffer and returns a pointer to
+    /// it in the buffer.
+    ///
+    /// The value is cloned and converted into a form suitable for placing into
+    /// a shared memory buffer by calling ToShmem::to_shmem on it.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn write<T: ToShmem>(&mut self, value: &T) -> std::result::Result<*mut T, String> {
+        // Reserve space for the value.
+        let dest: *mut T = self.alloc_value();
+
+        // Make a clone of the value with all of its heap allocations
+        // placed in the shared memory buffer.
+        let value = value.to_shmem(self)?;
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::write(dest, ManuallyDrop::into_inner(value));
+        }
+
+        // Return a pointer to the shared value.
+        Ok(dest)
+    }
+
+    /// Reserves space in the shared memory buffer to fit a value of type T,
+    /// and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc_value<T>(&mut self) -> *mut T {
+        self.alloc(Layout::new::<T>())
+    }
+
+    /// Reserves space in the shared memory buffer to fit an array of values of
+    /// type T, and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc_array<T>(&mut self, len: usize) -> *mut T {
+        if len == 0 {
+            return NonNull::dangling().as_ptr();
+        }
+
+        let size = mem::size_of::<T>();
+        let align = mem::align_of::<T>();
+
+        self.alloc(Layout::from_size_align(padded_size(size, align) * len, align).unwrap())
+    }
+
+    /// Reserves space in the shared memory buffer that conforms to the
+    /// specified layout, and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc<T>(&mut self, layout: Layout) -> *mut T {
+        // Amount of padding to align the value.
+        //
+        // The addition can't overflow, since self.index <= self.capacity, and
+        // for us to have successfully allocated the buffer, `buffer + capacity`
+        // can't overflow.
+        let padding = padding_needed_for(self.buffer as usize + self.index, layout.align());
+
+        // Reserve space for the padding.
+        let start = self.index.checked_add(padding).unwrap();
+        assert!(start <= std::isize::MAX as usize); // for the cast below
+
+        // Reserve space for the value.
+        let end = start.checked_add(layout.size()).unwrap();
+        assert!(end <= self.capacity);
+
+        self.index = end;
+        unsafe { self.buffer.add(start) as *mut T }
+    }
+}
+
+/// A type that can be copied into a SharedMemoryBuilder.
+pub trait ToShmem: Sized {
+    /// Clones this value into a form suitable for writing into a
+    /// SharedMemoryBuilder.
+    ///
+    /// If this value owns any heap allocations, they should be written into
+    /// `builder` so that the return value of this function can point to the
+    /// copy in the shared memory buffer.
+    ///
+    /// The return type is wrapped in ManuallyDrop to make it harder to
+    /// accidentally invoke the destructor of the value that is produced.
+    ///
+    /// Returns a Result so that we can gracefully recover from unexpected
+    /// content.
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self>;
+}
+
+#[macro_export]
+macro_rules! impl_trivial_to_shmem {
+    ($($ty:ty),*) => {
+        $(
+            impl $crate::ToShmem for $ty {
+                fn to_shmem(
+                    &self,
+                    _builder: &mut $crate::SharedMemoryBuilder,
+                ) -> $crate::Result<Self> {
+                    $crate::Result::Ok(::std::mem::ManuallyDrop::new(*self))
+                }
+            }
+        )*
+    };
+}
+
+impl_trivial_to_shmem!(
+    (),
+    bool,
+    f32,
+    f64,
+    i8,
+    i16,
+    i32,
+    i64,
+    u8,
+    u16,
+    u32,
+    u64,
+    isize,
+    usize
+);
+
+impl_trivial_to_shmem!(cssparser::RGBA);
+impl_trivial_to_shmem!(cssparser::SourceLocation);
+impl_trivial_to_shmem!(cssparser::TokenSerializationType);
+
+impl<T> ToShmem for PhantomData<T> {
+    fn to_shmem(&self, _builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        Ok(ManuallyDrop::new(*self))
+    }
+}
+
+impl<T: ToShmem> ToShmem for Range<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        Ok(ManuallyDrop::new(Range {
+            start: ManuallyDrop::into_inner(self.start.to_shmem(builder)?),
+            end: ManuallyDrop::into_inner(self.end.to_shmem(builder)?),
+        }))
+    }
+}
+
+impl ToShmem for cssparser::UnicodeRange {
+    fn to_shmem(&self, _builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        Ok(ManuallyDrop::new(cssparser::UnicodeRange {
+            start: self.start,
+            end: self.end,
+        }))
+    }
+}
+
+impl<T: ToShmem, U: ToShmem> ToShmem for (T, U) {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        Ok(ManuallyDrop::new((
+            ManuallyDrop::into_inner(self.0.to_shmem(builder)?),
+            ManuallyDrop::into_inner(self.1.to_shmem(builder)?),
+        )))
+    }
+}
+
+impl<T: ToShmem> ToShmem for Wrapping<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        Ok(ManuallyDrop::new(Wrapping(ManuallyDrop::into_inner(
+            self.0.to_shmem(builder)?,
+        ))))
+    }
+}
+
+impl<T: ToShmem> ToShmem for Box<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        // Reserve space for the boxed value.
+        let dest: *mut T = builder.alloc_value();
+
+        // Make a clone of the boxed value with all of its heap allocations
+        // placed in the shared memory buffer.
+        let value = (**self).to_shmem(builder)?;
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::write(dest, ManuallyDrop::into_inner(value));
+
+            Ok(ManuallyDrop::new(Box::from_raw(dest)))
+        }
+    }
+}
+
+/// Converts all the items in `src` into shared memory form, writes them into
+/// the specified buffer, and returns a pointer to the slice.
+unsafe fn to_shmem_slice_ptr<'a, T, I>(
+    src: I,
+    dest: *mut T,
+    builder: &mut SharedMemoryBuilder,
+) -> std::result::Result<*mut [T], String>
+where
+    T: 'a + ToShmem,
+    I: ExactSizeIterator<Item = &'a T>,
+{
+    let dest = slice::from_raw_parts_mut(dest, src.len());
+
+    // Make a clone of each element from the iterator with its own heap
+    // allocations placed in the buffer, and copy that clone into the buffer.
+    for (src, dest) in src.zip(dest.iter_mut()) {
+        ptr::write(dest, ManuallyDrop::into_inner(src.to_shmem(builder)?));
+    }
+
+    Ok(dest)
+}
+
+/// Writes all the items in `src` into a slice in the shared memory buffer and
+/// returns a pointer to the slice.
+pub unsafe fn to_shmem_slice<'a, T, I>(
+    src: I,
+    builder: &mut SharedMemoryBuilder,
+) -> std::result::Result<*mut [T], String>
+where
+    T: 'a + ToShmem,
+    I: ExactSizeIterator<Item = &'a T>,
+{
+    let dest = builder.alloc_array(src.len());
+    to_shmem_slice_ptr(src, dest, builder)
+}
+
+impl<T: ToShmem> ToShmem for Box<[T]> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        unsafe {
+            let dest = to_shmem_slice(self.iter(), builder)?;
+            Ok(ManuallyDrop::new(Box::from_raw(dest)))
+        }
+    }
+}
+
+impl ToShmem for Box<str> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        // Reserve space for the string bytes.
+        let dest: *mut u8 = builder.alloc_array(self.len());
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::copy(self.as_ptr(), dest, self.len());
+
+            Ok(ManuallyDrop::new(Box::from_raw(
+                str::from_utf8_unchecked_mut(slice::from_raw_parts_mut(dest, self.len())),
+            )))
+        }
+    }
+}
+
+impl ToShmem for String {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        // Reserve space for the string bytes.
+        let dest: *mut u8 = builder.alloc_array(self.len());
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::copy(self.as_ptr(), dest, self.len());
+
+            Ok(ManuallyDrop::new(String::from_raw_parts(
+                dest,
+                self.len(),
+                self.len(),
+            )))
+        }
+    }
+}
+
+impl ToShmem for CString {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        let len = self.as_bytes_with_nul().len();
+
+        // Reserve space for the string bytes.
+        let dest: *mut c_char = builder.alloc_array(len);
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::copy(self.as_ptr(), dest, len);
+
+            Ok(ManuallyDrop::new(CString::from_raw(dest)))
+        }
+    }
+}
+
+impl<T: ToShmem> ToShmem for Vec<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        unsafe {
+            let dest = to_shmem_slice(self.iter(), builder)? as *mut T;
+            let dest_vec = Vec::from_raw_parts(dest, self.len(), self.len());
+            Ok(ManuallyDrop::new(dest_vec))
+        }
+    }
+}
+
+impl<T: ToShmem, A: Array<Item = T>> ToShmem for SmallVec<A> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        let dest_vec = unsafe {
+            if self.spilled() {
+                // Place the items in a separate allocation in the shared memory
+                // buffer.
+                let dest = to_shmem_slice(self.iter(), builder)? as *mut T;
+                SmallVec::from_raw_parts(dest, self.len(), self.len())
+            } else {
+                // Place the items inline.
+                let mut s = SmallVec::new();
+                to_shmem_slice_ptr(self.iter(), s.as_mut_ptr(), builder)?;
+                s.set_len(self.len());
+                s
+            }
+        };
+
+        Ok(ManuallyDrop::new(dest_vec))
+    }
+}
+
+impl<T: ToShmem> ToShmem for Option<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        let v = match self {
+            Some(v) => Some(ManuallyDrop::into_inner(v.to_shmem(builder)?)),
+            None => None,
+        };
+
+        Ok(ManuallyDrop::new(v))
+    }
+}
+
+impl<T: ToShmem> ToShmem for Arc<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        // Assert that we don't encounter any shared references to values we
+        // don't expect.
+        #[cfg(debug_assertions)]
+        assert!(
+            !builder.shared_values.contains(&self.heap_ptr()),
+            "ToShmem failed for Arc<{}>: encountered a value with multiple \
+            references.",
+            std::any::type_name::<T>()
+        );
+
+        // Make a clone of the Arc-owned value with all of its heap allocations
+        // placed in the shared memory buffer.
+        let value = (**self).to_shmem(builder)?;
+
+        // Create a new Arc with the shared value and have it place its
+        // ArcInner in the shared memory buffer.
+        unsafe {
+            let static_arc = Arc::new_static(
+                |layout| builder.alloc(layout),
+                ManuallyDrop::into_inner(value),
+            );
+
+            #[cfg(debug_assertions)]
+            builder.shared_values.insert(self.heap_ptr());
+
+            Ok(ManuallyDrop::new(static_arc))
+        }
+    }
+}
+
+impl<H: ToShmem, T: ToShmem> ToShmem for ThinArc<H, T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        // We don't currently have any shared ThinArc values in stylesheets,
+        // so don't support them for now.
+        #[cfg(debug_assertions)]
+        assert!(
+            !builder.shared_values.contains(&self.heap_ptr()),
+            "ToShmem failed for ThinArc<T>: encountered a value with multiple references, which \
+             is not currently supported",
+        );
+
+        // Make a clone of the Arc-owned header and slice values with all of
+        // their heap allocations placed in the shared memory buffer.
+        let header = self.header.header.to_shmem(builder)?;
+        let mut values = Vec::with_capacity(self.slice.len());
+        for v in self.slice.iter() {
+            values.push(v.to_shmem(builder)?);
+        }
+
+        // Create a new ThinArc with the shared value and have it place
+        // its ArcInner in the shared memory buffer.
+        unsafe {
+            let static_arc = ThinArc::static_from_header_and_iter(
+                |layout| builder.alloc(layout),
+                ManuallyDrop::into_inner(header),
+                values.into_iter().map(ManuallyDrop::into_inner),
+            );
+
+            #[cfg(debug_assertions)]
+            builder.shared_values.insert(self.heap_ptr());
+
+            Ok(ManuallyDrop::new(static_arc))
+        }
+    }
+}
+
+impl ToShmem for SmallBitVec {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        let storage = match self.clone().into_storage() {
+            InternalStorage::Spilled(vs) => {
+                // Reserve space for the boxed slice values.
+                let len = vs.len();
+                let dest: *mut usize = builder.alloc_array(len);
+
+                unsafe {
+                    // Copy the value into the buffer.
+                    let src = vs.as_ptr() as *const usize;
+                    ptr::copy(src, dest, len);
+
+                    let dest_slice =
+                        Box::from_raw(slice::from_raw_parts_mut(dest, len) as *mut [usize]);
+                    InternalStorage::Spilled(dest_slice)
+                }
+            },
+            InternalStorage::Inline(x) => InternalStorage::Inline(x),
+        };
+        Ok(ManuallyDrop::new(unsafe {
+            SmallBitVec::from_storage(storage)
+        }))
+    }
+}
+
+#[cfg(feature = "string_cache")]
+impl<Static: string_cache::StaticAtomSet> ToShmem for string_cache::Atom<Static> {
+    fn to_shmem(&self, _: &mut SharedMemoryBuilder) -> Result<Self> {
+        // NOTE(emilio): In practice, this can be implemented trivially if
+        // string_cache could expose the implementation detail of static atoms
+        // being an index into the static table (and panicking in the
+        // non-static, non-inline cases).
+        unimplemented!(
+            "If servo wants to share stylesheets across processes, \
+             then ToShmem for Atom needs to be implemented"
+        )
+    }
+}