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diff --git a/servo/components/style/applicable_declarations.rs b/servo/components/style/applicable_declarations.rs
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+++ b/servo/components/style/applicable_declarations.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/. */
+
+//! Applicable declarations management.
+
+use crate::properties::PropertyDeclarationBlock;
+use crate::rule_tree::{CascadeLevel, StyleSource};
+use crate::shared_lock::Locked;
+use crate::stylesheets::layer_rule::LayerOrder;
+use servo_arc::Arc;
+use smallvec::SmallVec;
+
+/// List of applicable declarations. This is a transient structure that shuttles
+/// declarations between selector matching and inserting into the rule tree, and
+/// therefore we want to avoid heap-allocation where possible.
+///
+/// In measurements on wikipedia, we pretty much never have more than 8 applicable
+/// declarations, so we could consider making this 8 entries instead of 16.
+/// However, it may depend a lot on workload, and stack space is cheap.
+pub type ApplicableDeclarationList = SmallVec<[ApplicableDeclarationBlock; 16]>;
+
+/// Blink uses 18 bits to store source order, and does not check overflow [1].
+/// That's a limit that could be reached in realistic webpages, so we use
+/// 24 bits and enforce defined behavior in the overflow case.
+///
+/// Note that right now this restriction could be lifted if wanted (because we
+/// no longer stash the cascade level in the remaining bits), but we keep it in
+/// place in case we come up with a use-case for them, lacking reports of the
+/// current limit being too small.
+///
+/// [1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/
+///     RuleSet.h?l=128&rcl=90140ab80b84d0f889abc253410f44ed54ae04f3
+const SOURCE_ORDER_BITS: usize = 24;
+const SOURCE_ORDER_MAX: u32 = (1 << SOURCE_ORDER_BITS) - 1;
+const SOURCE_ORDER_MASK: u32 = SOURCE_ORDER_MAX;
+
+/// The cascade-level+layer order of this declaration.
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, PartialEq)]
+pub struct CascadePriority {
+    cascade_level: CascadeLevel,
+    layer_order: LayerOrder,
+}
+
+const_assert_eq!(
+    std::mem::size_of::<CascadePriority>(),
+    std::mem::size_of::<u32>()
+);
+
+impl PartialOrd for CascadePriority {
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for CascadePriority {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.cascade_level.cmp(&other.cascade_level).then_with(|| {
+            let ordering = self.layer_order.cmp(&other.layer_order);
+            if ordering == std::cmp::Ordering::Equal {
+                return ordering;
+            }
+            // https://drafts.csswg.org/css-cascade-5/#cascade-layering
+            //
+            //     Cascade layers (like declarations) are ordered by order
+            //     of appearance. When comparing declarations that belong to
+            //     different layers, then for normal rules the declaration
+            //     whose cascade layer is last wins, and for important rules
+            //     the declaration whose cascade layer is first wins.
+            //
+            // But the style attribute layer for some reason is special.
+            if self.cascade_level.is_important() &&
+                !self.layer_order.is_style_attribute_layer() &&
+                !other.layer_order.is_style_attribute_layer()
+            {
+                ordering.reverse()
+            } else {
+                ordering
+            }
+        })
+    }
+}
+
+impl CascadePriority {
+    /// Construct a new CascadePriority for a given (level, order) pair.
+    pub fn new(cascade_level: CascadeLevel, layer_order: LayerOrder) -> Self {
+        Self {
+            cascade_level,
+            layer_order,
+        }
+    }
+
+    /// Returns the layer order.
+    #[inline]
+    pub fn layer_order(&self) -> LayerOrder {
+        self.layer_order
+    }
+
+    /// Returns the cascade level.
+    #[inline]
+    pub fn cascade_level(&self) -> CascadeLevel {
+        self.cascade_level
+    }
+
+    /// Whether this declaration should be allowed if `revert` or `revert-layer`
+    /// have been specified on a given origin.
+    ///
+    /// `self` is the priority at which the `revert` or `revert-layer` keyword
+    /// have been specified.
+    pub fn allows_when_reverted(&self, other: &Self, origin_revert: bool) -> bool {
+        if origin_revert {
+            other.cascade_level.origin() < self.cascade_level.origin()
+        } else {
+            other.unimportant() < self.unimportant()
+        }
+    }
+
+    /// Convert this priority from "important" to "non-important", if needed.
+    pub fn unimportant(&self) -> Self {
+        Self::new(self.cascade_level().unimportant(), self.layer_order())
+    }
+
+    /// Convert this priority from "non-important" to "important", if needed.
+    pub fn important(&self) -> Self {
+        Self::new(self.cascade_level().important(), self.layer_order())
+    }
+}
+
+/// A property declaration together with its precedence among rules of equal
+/// specificity so that we can sort them.
+///
+/// This represents the declarations in a given declaration block for a given
+/// importance.
+#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
+pub struct ApplicableDeclarationBlock {
+    /// The style source, either a style rule, or a property declaration block.
+    #[ignore_malloc_size_of = "Arc"]
+    pub source: StyleSource,
+    /// The bits containing the source order, cascade level, and shadow cascade
+    /// order.
+    source_order: u32,
+    /// The specificity of the selector.
+    pub specificity: u32,
+    /// The cascade priority of the rule.
+    pub cascade_priority: CascadePriority,
+}
+
+impl ApplicableDeclarationBlock {
+    /// Constructs an applicable declaration block from a given property
+    /// declaration block and importance.
+    #[inline]
+    pub fn from_declarations(
+        declarations: Arc<Locked<PropertyDeclarationBlock>>,
+        level: CascadeLevel,
+        layer_order: LayerOrder,
+    ) -> Self {
+        ApplicableDeclarationBlock {
+            source: StyleSource::from_declarations(declarations),
+            source_order: 0,
+            specificity: 0,
+            cascade_priority: CascadePriority::new(level, layer_order),
+        }
+    }
+
+    /// Constructs an applicable declaration block from the given components.
+    #[inline]
+    pub fn new(
+        source: StyleSource,
+        source_order: u32,
+        level: CascadeLevel,
+        specificity: u32,
+        layer_order: LayerOrder,
+    ) -> Self {
+        ApplicableDeclarationBlock {
+            source,
+            source_order: source_order & SOURCE_ORDER_MASK,
+            specificity,
+            cascade_priority: CascadePriority::new(level, layer_order),
+        }
+    }
+
+    /// Returns the source order of the block.
+    #[inline]
+    pub fn source_order(&self) -> u32 {
+        self.source_order
+    }
+
+    /// Returns the cascade level of the block.
+    #[inline]
+    pub fn level(&self) -> CascadeLevel {
+        self.cascade_priority.cascade_level()
+    }
+
+    /// Returns the cascade level of the block.
+    #[inline]
+    pub fn layer_order(&self) -> LayerOrder {
+        self.cascade_priority.layer_order()
+    }
+
+    /// Convenience method to consume self and return the right thing for the
+    /// rule tree to iterate over.
+    #[inline]
+    pub fn for_rule_tree(self) -> (StyleSource, CascadePriority) {
+        (self.source, self.cascade_priority)
+    }
+}
+
+// Size of this struct determines sorting and selector-matching performance.
+size_of_test!(ApplicableDeclarationBlock, 24);