diff --git a/components/script/dom/cssstyledeclaration.rs b/components/script/dom/cssstyledeclaration.rs
index abd6d207dd1..37ff6e7a3bf 100644
--- a/components/script/dom/cssstyledeclaration.rs
+++ b/components/script/dom/cssstyledeclaration.rs
@@ -242,24 +242,26 @@ impl CSSStyleDeclarationMethods for CSSStyleDeclaration {
         }
 
         // Step 4
-        let priority = match &*priority {
+        let importance = match &*priority {
             "" => Importance::Normal,
             p if p.eq_ignore_ascii_case("important") => Importance::Important,
             _ => return Ok(()),
         };
 
-        let element = self.owner.upcast::<Element>();
+        let style_attribute = self.owner.style_attribute().borrow();
+        if let Some(ref lock) = *style_attribute {
+            let mut style_attribute = lock.write();
 
-        // Step 5 & 6
-        match Shorthand::from_name(&property) {
-            Some(shorthand) => {
-                element.set_inline_style_property_priority(shorthand.longhands(), priority)
+            // Step 5 & 6
+            match Shorthand::from_name(&property) {
+                Some(shorthand) => style_attribute.set_importance(shorthand.longhands(), importance),
+                None => style_attribute.set_importance(&[&*property], importance),
             }
-            None => element.set_inline_style_property_priority(&[&*property], priority),
-        }
 
-        let node = element.upcast::<Node>();
-        node.dirty(NodeDamage::NodeStyleDamaged);
+            self.owner.set_style_attr(style_attribute.to_css_string());
+            let node = self.owner.upcast::<Node>();
+            node.dirty(NodeDamage::NodeStyleDamaged);
+        }
         Ok(())
     }
 
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 67e0b549d56..621faa912aa 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -5,7 +5,7 @@
 //! Element nodes.
 
 use app_units::Au;
-use cssparser::{Color, ToCss};
+use cssparser::Color;
 use devtools_traits::AttrInfo;
 use dom::activation::Activatable;
 use dom::attr::{Attr, AttrHelpersForLayout};
@@ -735,15 +735,6 @@ impl Element {
 
     // this sync method is called upon modification of the style_attribute property,
     // therefore, it should not trigger subsequent mutation events
-    pub fn sync_property_with_attrs_style(&self) {
-        let style_str = if let &Some(ref declarations) = &*self.style_attribute().borrow() {
-            declarations.read().to_css_string()
-        } else {
-            String::new()
-        };
-        self.set_style_attr(style_str)
-    }
-
     pub fn set_style_attr(&self, new_value: String) {
         let mut new_style = AttrValue::String(new_value);
 
@@ -767,35 +758,6 @@ impl Element {
          self.attrs.borrow_mut().push(JS::from_ref(&attr));
     }
 
-    pub fn set_inline_style_property_priority(&self,
-                                              properties: &[&str],
-                                              new_importance: Importance) {
-        {
-            let mut inline_declarations = self.style_attribute().borrow_mut();
-            if let &mut Some(ref mut block) = &mut *inline_declarations {
-                let mut block = block.write();
-                let block = &mut *block;
-                let declarations = &mut block.declarations;
-                for &mut (ref declaration, ref mut importance) in declarations {
-                    if properties.iter().any(|p| declaration.name() == **p) {
-                        match (*importance, new_importance) {
-                            (Importance::Normal, Importance::Important) => {
-                                block.important_count += 1;
-                            }
-                            (Importance::Important, Importance::Normal) => {
-                                block.important_count -= 1;
-                            }
-                            _ => {}
-                        }
-                        *importance = new_importance;
-                    }
-                }
-            }
-        }
-
-        self.sync_property_with_attrs_style();
-    }
-
     pub fn serialize(&self, traversal_scope: TraversalScope) -> Fallible<DOMString> {
         let mut writer = vec![];
         match serialize(&mut writer,
diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index 2300634b559..25d48a1cb55 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -154,6 +154,23 @@ impl PropertyDeclarationBlock {
         }
     }
 
+    pub fn set_importance(&mut self, property_names: &[&str], new_importance: Importance) {
+        for &mut (ref declaration, ref mut importance) in &mut self.declarations {
+            if property_names.iter().any(|p| declaration.matches(p)) {
+                match (*importance, new_importance) {
+                    (Importance::Normal, Importance::Important) => {
+                        self.important_count += 1;
+                    }
+                    (Importance::Important, Importance::Normal) => {
+                        self.important_count -= 1;
+                    }
+                    _ => {}
+                }
+                *importance = new_importance;
+            }
+        }
+    }
+
     /// https://dev.w3.org/csswg/cssom/#dom-cssstyledeclaration-removeproperty
     pub fn remove_property(&mut self, property_name: &str) {
         // Step 2