See the other comment for a MITM large browser server: https://news.ycombinator.com/item?id=45446991

To do everything onboard, maybe, just maybe, you could parse basic HTML/CSS and images. But the majority of pages would of course fail anyway without full support for every modern feature.

This seems like it could be useful: https://limpet.net/mbrubeck/2014/08/08/toy-layout-engine-1.h...