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.

(cherry-picked from 10e654218a65ec9a0ffc875137cea945389c69f1)

Fixes #14836
Closes gh-1532