layout: Implement word-spacing per CSS 2.1 § 16.4.

This assumes that there are no ligatures that span across multiple
words. Since we have a per-word shape cache, this is a safe assumption
as of now. I have left comments to ensure that, if and when this is
revisted, we make sure to handle it properly.

components/style/properties/mod.rs.mako

@@ -1111,6 +1111,29 @@ pub mod longhands {
         }
     </%self:single_component_value>
 
+    <%self:single_component_value name="word-spacing">
+        pub type SpecifiedValue = Option<specified::Length>;
+        pub mod computed_value {
+            use super::super::Au;
+            pub type T = Option<Au>;
+        }
+        #[inline]
+        pub fn get_initial_value() -> computed_value::T {
+            None
+        }
+        #[inline]
+        pub fn to_computed_value(value: SpecifiedValue, context: &computed::Context)
+                                 -> computed_value::T {
+            value.map(|length| computed::compute_Au(length, context))
+        }
+        pub fn from_component_value(input: &ComponentValue, _: &Url) -> Result<SpecifiedValue,()> {
+            match input {
+                &Ident(ref value) if value.eq_ignore_ascii_case("normal") => Ok(None),
+                _ => specified::Length::parse_non_negative(input).map(|length| Some(length)),
+            }
+        }
+    </%self:single_component_value>
+
     ${predefined_type("text-indent", "LengthOrPercentage", "computed::LP_Length(Au(0))")}
 
     ${new_style_struct("Text", is_inherited=False)}