diff --git a/components/script/dom/cssrulelist.rs b/components/script/dom/cssrulelist.rs
index deb6b93e8b13..c10b9196092a 100644
--- a/components/script/dom/cssrulelist.rs
+++ b/components/script/dom/cssrulelist.rs
@@ -15,13 +15,23 @@ use dom::cssstylesheet::CSSStyleSheet;
 use dom::window::Window;
 use parking_lot::RwLock;
 use std::sync::Arc;
-use style::parser::ParserContextExtraData;
-use style::stylesheets::{CssRules, KeyframesRule, Origin};
+use style::stylesheets::{CssRules, KeyframesRule, RulesMutateError};
 use style::stylesheets::CssRule as StyleCssRule;
 
 no_jsmanaged_fields!(RulesSource);
 no_jsmanaged_fields!(CssRules);
 
+impl From<RulesMutateError> for Error {
+    fn from(other: RulesMutateError) -> Self {
+        match other {
+            RulesMutateError::Syntax => Error::Syntax,
+            RulesMutateError::IndexSize => Error::IndexSize,
+            RulesMutateError::HierarchyRequest => Error::HierarchyRequest,
+            RulesMutateError::InvalidState => Error::InvalidState,
+        }
+    }
+}
+
 #[dom_struct]
 pub struct CSSRuleList {
     reflector_: Reflector,
@@ -64,21 +74,9 @@ impl CSSRuleList {
                            CSSRuleListBinding::Wrap)
     }
 
-    /// https://drafts.csswg.org/cssom/#insert-a-css-rule
-    ///
     /// Should only be called for CssRules-backed rules. Use append_lazy_rule
     /// for keyframes-backed rules.
     pub fn insert_rule(&self, rule: &str, idx: u32, nested: bool) -> Fallible<u32> {
-        use style::stylesheets::SingleRuleParseError;
-        /// Insert an item into a vector, appending if it is out of bounds
-        fn insert<T>(vec: &mut Vec<T>, index: usize, item: T) {
-            if index >= vec.len() {
-                vec.push(item);
-            } else {
-                vec.insert(index, item);
-            }
-        }
-
         let css_rules = if let RulesSource::Rules(ref rules) = self.rules {
             rules
         } else {
@@ -90,58 +88,12 @@ impl CSSRuleList {
         let doc = window.Document();
         let index = idx as usize;
 
+        let new_rule = css_rules.insert_rule(rule, doc.url().clone(), index, nested)?;
 
-        let new_rule = {
-            let rules = css_rules.0.read();
-            let state = if nested {
-                None
-            } else {
-                Some(CssRules::state_at_index(&rules, index))
-            };
-
-            let rev_state = CssRules::state_at_index_rev(&rules, index);
-
-            // Step 1, 2
-            // XXXManishearth get url from correct location
-            // XXXManishearth should we also store the namespace map?
-            let parse_result = StyleCssRule::parse(&rule, Origin::Author,
-                                               doc.url().clone(),
-                                               ParserContextExtraData::default(),
-                                               state);
-
-            if let Err(SingleRuleParseError::Syntax) = parse_result {
-                return Err(Error::Syntax)
-            }
-
-            // Step 3, 4
-            if index > rules.len() {
-                return Err(Error::IndexSize);
-            }
-
-            let (new_rule, new_state) = try!(parse_result.map_err(|_| Error::HierarchyRequest));
-
-            if new_state > rev_state {
-                // We inserted a rule too early, e.g. inserting
-                // a regular style rule before @namespace rules
-                return Err((Error::HierarchyRequest));
-            }
-
-            // Step 6
-            if let StyleCssRule::Namespace(..) = new_rule {
-                if !CssRules::only_ns_or_import(&rules) {
-                    return Err(Error::InvalidState);
-                }
-            }
-
-            new_rule
-        };
-
-        insert(&mut css_rules.0.write(), index, new_rule.clone());
         let sheet = self.sheet.get();
         let sheet = sheet.as_ref().map(|sheet| &**sheet);
         let dom_rule = CSSRule::new_specific(&window, sheet, new_rule);
-        insert(&mut self.dom_rules.borrow_mut(),
-               index, MutNullableHeap::new(Some(&*dom_rule)));
+        self.dom_rules.borrow_mut().insert(index, MutNullableHeap::new(Some(&*dom_rule)));
         Ok((idx))
     }
 
diff --git a/components/style/stylesheets.rs b/components/style/stylesheets.rs
index 12ca0423cfa5..cce7e40c223d 100644
--- a/components/style/stylesheets.rs
+++ b/components/style/stylesheets.rs
@@ -60,6 +60,22 @@ impl From<Vec<CssRule>> for CssRules {
     }
 }
 
+pub enum RulesMutateError {
+    Syntax,
+    IndexSize,
+    HierarchyRequest,
+    InvalidState,
+}
+
+impl From<SingleRuleParseError> for RulesMutateError {
+    fn from(other: SingleRuleParseError) -> Self {
+        match other {
+            SingleRuleParseError::Syntax => RulesMutateError::Syntax,
+            SingleRuleParseError::Hierarchy => RulesMutateError::HierarchyRequest,
+        }
+    }
+}
+
 impl CssRules {
     // used in CSSOM
     pub fn only_ns_or_import(rules: &[CssRule]) -> bool {
@@ -102,6 +118,46 @@ impl CssRules {
             State::Body
         }
     }
+
+    /// https://drafts.csswg.org/cssom/#insert-a-css-rule
+    pub fn insert_rule(&self, rule: &str, base_url: ServoUrl, index: usize, nested: bool)
+                       -> Result<CssRule, RulesMutateError> {
+        let mut rules = self.0.write();
+
+        // Step 1, 2
+        if index > rules.len() {
+            return Err(RulesMutateError::IndexSize);
+        }
+
+        let state = if nested {
+            None
+        } else {
+            Some(CssRules::state_at_index(&rules, index))
+        };
+
+        // Step 3, 4
+        // XXXManishearth should we also store the namespace map?
+        let (new_rule, new_state) = try!(CssRule::parse(&rule, Origin::Author, base_url,
+                                                        ParserContextExtraData::default(), state));
+
+        // Step 5
+        let rev_state = CssRules::state_at_index_rev(&rules, index);
+        if new_state > rev_state {
+            // We inserted a rule too early, e.g. inserting
+            // a regular style rule before @namespace rules
+            return Err(RulesMutateError::HierarchyRequest);
+        }
+
+        // Step 6
+        if let CssRule::Namespace(..) = new_rule {
+            if !CssRules::only_ns_or_import(&rules) {
+                return Err(RulesMutateError::InvalidState);
+            }
+        }
+
+        rules.insert(index, new_rule.clone());
+        Ok(new_rule)
+    }
 }
 
 #[derive(Debug)]