[css-ui] Form control pseudo-element audit

[nt1m]

I started an audit of ::-webkit- pseudo-elements that can be used to style form controls: https://docs.google.com/spreadsheets/d/1an5v1KpapIGq0LzHd5UpTrUaJ-n08btQSmtEdWMDtFs/edit#gid=0

This spreadsheet only has the WebKit ones, not the Blink ones, but feel free to give me a list if you'd like those to be added. Ditto for Gecko.

I want to start this issue so we can collectively figure out which of these are highest value to standardize, so we can start filing individual issues for those and bikeshed naming/structure there.

[nt1m]

cc @emilio @mfreed7 @josepharhar @pxlcoder @annevk @fantasai

[mfreed7]

@lilles

[tabatkins]

For those of us without easy access to an Apple device, can we get a little more context on what some of these are? For example, I'm not sure what the auto-fill ones are, what the scope of some of the datetime ones are, or what several of the search ones are.