Document supported CSS properties

[nicoburns]

We should consider creating a table like https://css.tobyase.de/ That website is open source so we could probably fork it.

We may also want to document which values we support for each property.

Some examples we could take inspiration from when producing a feature support table:

• Dropflow (https://github.com/chearon/dropflow#supported-css-rules)
• Ladybird (https://css.tobyase.de/)
• Sciter (https://sciter.com/docs/content/css/cssmap.html)
• litehtml (https://docs.google.com/spreadsheets/d/1CM6p2BU0XwM7KS0ZUIP7u25g2rrHXKx_ANGdDG98vwc)
• NetSurf (https://www.netsurf-browser.org/documentation/progress)