script: Don't avoid all the mutation notification methods when the style attribute changes.

Styling was correct because of the explicit dirtiness, but still not fun.

Some things, like dynamic updates to with things like [style~="color"] ~ foo
selectors, were pretty broken, because we didn't take snapshots of those
attributes.

components/style/attr.rs

@@ -11,9 +11,12 @@ use app_units::Au;
 use cssparser::{self, Color, RGBA};
 use euclid::num::Zero;
 use num_traits::ToPrimitive;
+use parking_lot::RwLock;
+use properties::PropertyDeclarationBlock;
 use servo_url::ServoUrl;
 use std::ascii::AsciiExt;
 use std::str::FromStr;
+use std::sync::Arc;
 use str::{HTML_SPACE_CHARACTERS, read_exponent, read_fraction};
 use str::{read_numbers, split_commas, split_html_space_chars};
 #[cfg(not(feature = "gecko"))] use str::str_join;
@@ -30,7 +33,7 @@ pub enum LengthOrPercentageOrAuto {
     Length(Au),
 }
 
-#[derive(PartialEq, Clone, Debug)]
+#[derive(Clone, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum AttrValue {
     String(String),
@@ -43,6 +46,22 @@ pub enum AttrValue {
     Color(String, Option<RGBA>),
     Dimension(String, LengthOrPercentageOrAuto),
     Url(String, Option<ServoUrl>),
+
+    /// Note that this variant is only used transitively as a fast path to set
+    /// the property declaration block relevant to the style of an element when
+    /// set from the inline declaration of that element (that is,
+    /// `element.style`).
+    ///
+    /// This can, as of this writing, only correspond to the value of the
+    /// `style` element, and is set from its relevant CSSInlineStyleDeclaration,
+    /// and then converted to a string in Element::attribute_mutated.
+    ///
+    /// Note that we don't necessarily need to do that (we could just clone the
+    /// declaration block), but that avoids keeping a refcounted
+    /// declarationblock for longer than needed.
+    Declaration(String,
+                #[cfg_attr(feature = "servo", ignore_heap_size_of = "Arc")]
+                Arc<RwLock<PropertyDeclarationBlock>>)
 }
 
 /// Shared implementation to parse an integer according to
@@ -330,6 +349,7 @@ impl ::std::ops::Deref for AttrValue {
                 AttrValue::Color(ref value, _) |
                 AttrValue::Int(ref value, _) |
                 AttrValue::Url(ref value, _) |
+                AttrValue::Declaration(ref value, _) |
                 AttrValue::Dimension(ref value, _) => &value,
             AttrValue::Atom(ref value) => &value,
         }