add css support list in tree

[metajack]

We should move the data for https://docs.google.com/spreadsheets/d/1CxLS8w8GwK-2euVErrqpUUb76PiZa6w5h5EnGsL9KFs/edit?usp=sharing to an in tree list, and then generate a nice html display like we do doc generation. This will allow us to keep it up to date as new PRs land.

Additionally, element.style things only exist for things we actually support, but it would be nice not to maintain the list of supported features in two places. We should come up with a way to generate one from the other.

[metajack]

cc @SimonSapin @jdm @yichoi

[SimonSapin]

At the moment the Mako template can only generate one (.rs) file because that was easier, but can be extended to generate more stuff automatically and we definitely should not duplicate things like the list of supported properties.

I’ve been looking at Cargo’s new build script system anyway (rust-lang/cargo#824, rust-lang/rust#18849, rust-lang/rust#18810 (comment)) to make the Mako template use it and avoid having generated files in the source tree.

In both cases though (doc of supported properties and element.style), the caveat from @pcwalton’s email apply:

This only indicates whether Servo has any support at all for each CSS property. It does not test whether the implementation of the property is bug-free in Servo, whether all of the features of the property (e.g. multiple backgrounds) are supported, or whether the property is implemented in a performant manner.

[mbrubeck]

Fixed by #10208.