The following properties accept <alpha-value> [1], which is
"<number> | <percentage>", so we update the parser, spec links, and
their web-platform-tests.
- opacity
- flood-opacity
- fill-opacity
- stroke-opacity
- stop-opacity
- -moz-window-opacity
Besides, shape-image-threshold [2] still only accepts <number>, so we need
to support a different version of Opacity::parse().
[1] https://drafts.csswg.org/css-color/#typedef-alpha-value
[2] https://drafts.csswg.org/css-shapes/#shape-image-threshold-property