Loading CSS before feature detecting

[Swaagie]

Loading CSS is currently broken directly after page load, but before any other stylesheets are loaded or feature detection is done. Which also happens async through adding a stylesheet. Either .style or add needs to wait for feature detection to be done. However there is no finite end to feature detection atm.

Also the fallback based on #id height change detection is broken. As this assumes the height change to be present in the CSS, which was there for BigPipe but not for general purpose