CSP - support for strict style-src rule

[lakshit-dua]

Preprocessing of styles currently utilizes insertRule function to add rules to stylesheets. Moreover style object is appended to the head of the document. These don't play well with strict style-src CSP rules.

Are there any known workarounds for this?

[alexcjohnson]

Discussed in #2355 (ancient issue but it hasn't yet become a top priority)