style: Introduce css(parse_condition).

This will allow us to add a pref for this, and to parse it only on chrome easily.

Bug: 1288572
Reviewed-by: xidorn
MozReview-Commit-ID: L1rsyc2A2hu

1 files changed, 11 insertions, 2 deletions

diff --git a/components/style/parser.rs b/components/style/parser.rs
index 26f4aeea071..1b7c65e2559 100644
--- a/components/style/parser.rs
+++ b/components/style/parser.rs
@@ -150,10 +150,19 @@ impl<'a> ParserContext<'a> {
     }
 }
 
-// XXXManishearth Replace all specified value parse impls with impls of this
-// trait. This will make it easy to write more generic values in the future.
 /// A trait to abstract parsing of a specified value given a `ParserContext` and
 /// CSS input.
+///
+/// This can be derived on keywords with `#[derive(Parse)]`.
+///
+/// The derive code understands the following attributes on each of the variants:
+///
+///  * `#[css(aliases = "foo,bar")]` can be used to alias a value with another
+///    at parse-time.
+///
+///  * `#[css(parse_condition = "function")]` can be used to make the parsing of
+///    the value conditional on `function`, which will be invoked with a
+///    `&ParserContext` reference.
 pub trait Parse: Sized {
     /// Parse a value of this type.
     ///