This is already occurred, in some limited from. See quicks mode [1], for instance. Also there is WebAssembly, which should be the replacement for JS. At some point a good-enough UI toolkit for it will be written, and then that could replace HTML/CSS.
For the younger greenhorns among us: Once upon a time, we used to declare what version of HTML a page was written in with the doctype tag at the very top of the file.
Certain versions of HTML thus declared, incorrectly formed doctypes, or more often the absence of the doctype declaration altogether, would tell most rendering engines to enter Quirks Mode and render the page with backwards compatibility as the highest priority.
Certain versions of HTML thus declared, incorrectly formed doctypes, or more often the absence of the doctype declaration altogether, would tell most rendering engines to enter Quirks Mode and render the page with backwards compatibility as the highest priority.