Using px for root nullifies browser font-size

Created by: keithjgrant

I know this has been hashed over in the past, but I don't see any discussion about it for v4. Using px for the root font-size invalidates the user's font-size settings in the browser. This really should be defined in em/rem/%. The thought that bootstrap will be stripping away this functionality from users on a large portion of the web is unsettling.

Yes, I know that zoom still works, but this is a temporary, per-tab setting. Users with low vision are the one's most likely to need to increase their font size, and they will need it increased all the time, permanently. The shouldn't have to zoom in again every time they open a new tab.

Rems are broadly supported now. Use those on the root element. I see no benefit to favoring pixels on the root, particularly when the framework uses ems and rems liberally everywhere else.