Web bundles is supposed to replace it, cf. https://web.dev/web-bundles/

Interestingly, the support for mht/multipart is still there in Chrome and Firefox; or at least it was relatively recently. However it only works for files and not when accessing an mht file over http. The format itself is relatively simple.

Yeah it's pretty much the MIME email format as a container for a webpage.

A lot of the issues "solved" in modern frameworks could have been addressed by using this, but instead things went a different path.