Polyfilling CSS with CSS Parser Extensions

In April I attended #BlinkOn, the conference for web platform contributors in the Chromium open source project. At the conference I gave a presentation about “CSS Parser Extensions”, a wild idea I have to fix CSS polyfilling once and for all.

If you didn’t know, polyfilling CSS features is extremely hard, mainly because the CSS Parser discards what it does not understand. So what if, instead of having authors write their own parser and cascade to polyfill a CSS feature, they could teach the parser some new tricks?