diff --git a/components/style/rule_tree/core.rs b/components/style/rule_tree/core.rs
index d1c8bda5379..ae1ba7bed94 100644
--- a/components/style/rule_tree/core.rs
+++ b/components/style/rule_tree/core.rs
@@ -202,6 +202,9 @@ impl RuleTree {
 /// where it likely did not result from a rigorous performance analysis.)
 const RULE_TREE_GC_INTERVAL: usize = 300;
 
+/// Used for some size assertions.
+pub const RULE_NODE_SIZE: usize = std::mem::size_of::<RuleNode>();
+
 /// A node in the rule tree.
 struct RuleNode {
     /// The root node. Only the root has no root pointer, for obvious reasons.
diff --git a/components/style/rule_tree/mod.rs b/components/style/rule_tree/mod.rs
index ab5273db781..e50382255ca 100644
--- a/components/style/rule_tree/mod.rs
+++ b/components/style/rule_tree/mod.rs
@@ -19,7 +19,7 @@ mod map;
 mod source;
 mod unsafe_box;
 
-pub use self::core::{RuleTree, StrongRuleNode};
+pub use self::core::{RuleTree, StrongRuleNode, RULE_NODE_SIZE};
 pub use self::level::{CascadeLevel, ShadowCascadeOrder};
 pub use self::source::StyleSource;