> Again, the media query only does its thing when the browser says JavaScript isn't supported

I get how it works, it just feels like a hack designed to work around a missing feature, rather than the ideal way to handle the problem. Phrased another way: if the scripting media query was ubiquitous I would never go back to toggling classes via JS.

> Another factor may have been IE;

Browsers have been shipping features that don't work in IE for the better part of the last decade. If you think CSS nesting would break IE, imagine what IE would do when it encounters a modern JS feature that can't be polyfilled like Proxies.