One of topcoat's goals is to be highly performant CSS (especially on mobile, it's hard to deliver fast UI). To this end they're using the Chromium Telemetry project to benchmark the performance of their CSS continually. Basically CSS Performance Regression Testing.

See http://bench.topcoat.io/ and http://bench.topcoat.io/v2/view/results?date=30&; (metrics for time spent in recalc styles, layout, paint, composite layers)