1 files changed, 9 insertions, 0 deletions
diff --git a/components/style/selector_matching.rs b/components/style/selector_matching.rs
index 211ae3dbe56..a9c0839f5e8 100644
--- a/components/style/selector_matching.rs
+++ b/components/style/selector_matching.rs
@@ -17,6 +17,7 @@ use media_queries::Device;
 use node::TElementAttributes;
 use properties::{PropertyDeclaration, PropertyDeclarationBlock};
 use stylesheets::{Stylesheet, CSSRuleIteratorExt, Origin};
+use viewport::{ViewportConstraints, ViewportRuleCascade};
 
 
 pub type DeclarationBlock = GenericDeclarationBlock<Vec<PropertyDeclaration>>;
@@ -69,6 +70,14 @@ impl Stylist {
         stylist
     }
 
+    pub fn constrain_viewport(&self) -> Option<ViewportConstraints> {
+        let cascaded_rule = self.stylesheets.iter()
+            .flat_map(|s| s.effective_rules(&self.device).viewport())
+            .cascade();
+
+        ViewportConstraints::maybe_new(self.device.viewport_size, &cascaded_rule)
+    }
+
     pub fn update(&mut self) -> bool {
         if self.is_dirty {
             self.element_map = PerPseudoElementSelectorMap::new();