Thanks for your post, it gave me an idea. I remember this write up on inserting code into Go to do tracing, the code was called GoWeave https://github.com/deferpanic/goweave

My thought was, what if we used something like SASS to generate the css. We then automatically inserted some type of numbered 1 pixel gif background image rules into each aspect of the SASS, ran the page though some automated browser/mobile tests, had the server record formatted logs for these 1 pixel gifs, then finally created a detailed report that had the mapping of 1 pixel gifs back to the sass/css aspect.