Css: Revert 24e587929f62428e1959b10aace6dc4fd65ab397

The workaround to be able to change !important styles broke the browser
keeping the old CSS value if the new one was rejected. Patching it would
involve a significant perf hit (~33%) so the initial patch needs to be
reverted instead.

Tests by m_gol & gibson042.

Fixes #14836
Closes gh-1532
Css: Fix tests

It's not easy to find a CSS property that is implemented even in
ancient browsers and that returns a consistent result among browsers
when passed through .css(). color didn't work since Firefox normalizes
it an empty value to rgb(0, 0, 0).