style: Support the all shorthand.

Fixes #15055.

components/script/dom/webidls/CSSStyleDeclaration.webidl

@@ -33,6 +33,8 @@ interface CSSStyleDeclaration {
 };
 
 partial interface CSSStyleDeclaration {
+  [SetterThrows, TreatNullAs=EmptyString] attribute DOMString all;
+
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString background;
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString backgroundColor;
   [SetterThrows, TreatNullAs=EmptyString] attribute DOMString background-color;

components/style/properties/properties.mako.rs

@@ -165,6 +165,29 @@ pub mod shorthands {
     <%include file="/shorthand/position.mako.rs" />
     <%include file="/shorthand/inherited_svg.mako.rs" />
     <%include file="/shorthand/text.mako.rs" />
+
+    // FIXME(heycam): It might be worth trying to handle this more efficiently
+    // as part of the cascade function, rather than expanding this all
+    // shorthand out to hundreds of specified values.
+    <% components_of_all_shorthand = [p.name for p in data.longhands if p.name not in ['direction', 'unicode-bidi']] %>
+    <%helpers:shorthand name="all"
+                        sub_properties="${' '.join(components_of_all_shorthand)}"
+                        spec="https://drafts.csswg.org/css-cascade-3/#all-shorthand">
+
+        pub fn parse_value(_context: &ParserContext, _input: &mut Parser) -> Result<Longhands, ()> {
+            // cascade_property handles parsing the CSS-wide keywords, so we
+            // don't need to handle any other values here.
+            Err(())
+        }
+
+        impl<'a> ToCss for LonghandsToSerialize<'a> {
+            fn to_css<W>(&self, _dest: &mut W) -> fmt::Result where W: fmt::Write {
+                // get_shorthand_appendable_value handles this for us.
+                debug_assert!(false, "we shouldn't need to be called");
+                Ok(())
+            }
+        }
+    </%helpers:shorthand>
 }
 
 /// A module with all the code related to animated properties.