Tests: Blacklist beforeunload test in iOS

iOS has the window.onbeforeunload field but doesn't support the beforeunload
handler making it impossible to feature-detect the support.

(cherry-picked from 988d99ad278c2c9dc16c68f86bddc4df99dcd928)

Fixes #14717
Closes gh-1496
Css: Document reliableMarginRight is needed for Android 2.3
Css: change boxSizingReliable in IE<9.

Change boxSizingReliable test value to false in all IE versions.
Modify support comments to argument getComputedStyle guards
with oldIE support and not node.js which is not supported in 1.x.