Option to only use insertRule in certain environments

[bslinger]

I appreciate the performance increase of adding new styles using CSSStyleSheet.insertRule, however I've run into issues when using visual testing tools such as Percy as they take a snapshot of the current DOM and render it elsewhere, meaning the styles added during execution aren't available.

Presumably this would need to be configurable in some way, but ideally I would like to use insertRule in production and append to the <style> tag in other environments.

Anybody have any thoughts on the best way to approach this?