[jeroenhd]

I don't think this matter much? It's just a performance optimization, Safari can remain slow while all modern browsers benefit. Unknown CSS values should be ignored so there should be no impact on actual usability.

Seems to me like it's excellent for daily use despite the lack of compatibility of outdated browsers.

[sebazzz]

It matters because it causes a new stacking context. Z-index only affects elements within the same stacking context, so you then can get weird behaviour that one floating tooltip overlaps something in Firefox/Chrome but doesn't in Safari because Safari doesn't create the stacking context.