Proposal: remove valid color checking from PWA requirements

Summary

So after https://github.com/GoogleChrome/lighthouse/issues/3891#issuecomment-525929813 I went to update our cssstyle dependency to fix the rgba(0,0,0,0) bug, but then it turns out the new version that fixes that introduces several other bugs. The valid color checking has been a whack-a-mole problem for years and it feels not very useful so I was curious what it looks like on real sites.

According to July HTTPArchive data…

Only .08% of sites ever trigger our invalid color logic and of that set ~42% are actually valid colors that we incorrectly flag as invalid (see trix).

Proposal A: Change invalid color check to a warning on PWA themed-omnibox and splash-screen. It feels like this is a warning-level thing anyway and we’re almost wrong as often as we’re right.

Proposal B: Remove invalid color check entirely. 20KB for something useful to .05% of sites feels like a bit much, false warnings aren’t great either.

[patrickhulce]

We’ve decided to move forward with dropping the color checking here.

[connorjclark]

We could close the gap between implementation and our color validation by not writing our own color validation (or rather, relying on cssstlye to be correct).

when the color is invalid, blink pushes an error to the parser: https://cs.chromium.org/chromium/src/third_party/blink/renderer/modules/manifest/manifest_parser.cc?l=164&rcl=65b3e11ce92ca41af626cb68ec84b795c90453ad

These end up as console errors: https://cs.chromium.org/chromium/src/third_party/blink/renderer/modules/manifest/manifest_manager.cc?l=194&rcl=ab4ecdb6a2dacf0a35344c2baa9509a9659eec86