Implement keyword values in border-width

[Manishearth]

Please see the property hacking guide first. Familiarity with Rust required. This is not implementing a new property, just extending an existing one.

For border-foo-width (e.g. border-top-width), we currently support length values. However, this struct supports keywords too.

We currently parse it as <length>, but the full syntax is <length> | thin | medium | thick. Now, the keyword values map to integer lengths (Firefox maps them to 1px, 3px and 5px respectively, you can use Length::Absolute(1).to_computed_value(context) to get these in Au). So you will have to change the SpecifiedValue struct so that it can store thin/medium/thick keyword values (as well as lengths), and change to_computed_value so that it replaces the keyword values with the stuff specified.

You should be able to test this in Servo directly, just create a div and give it different keyword borders.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[Manishearth]

@heycam Gecko stores the constants 1/3/5 in the pres context. Should we be storing them in the computed value context (and fetching them from Gecko in stylo mode)? For this bug I'm just hardcoding them, but we might eventually want to do something else.

[aethanyc]

I would like to work on this.

[Manishearth]

Great! Let us know if you have questions!

[heycam]

Hard coding is fine for now. For Gecko, we probably should just change them to be constants defined somewhere rather than exposed through the nsPresContext.

[aethanyc]

Any bug filed on gecko for moving those border-width constants from nsPresContext to elsewhere?

[heycam]

There isn't AFAIK. Please feel free to file one!

[Manishearth]

https://bugzilla.mozilla.org/show_bug.cgi?id=1312155