Add support for @layer #17
Description
This has an existing pull request : #7
- specification : https://drafts.csswg.org/css-cascade-5/#layering
- wpt : https://wpt.fyi/results/css/css-cascade?label=master&label=experimental&aligned&q=layer
- mdn docs : https://developer.mozilla.org/en-US/docs/Web/CSS/@layer
- shipped in all browsers
This feature is very tricky to implement safely without a working test suite