Yes in theory, you can upload an html file entirely. There are no limits to file formats.
Directly no, you can’t modifiy an uploaded file on IPFS.
However there are a few tricks, for example if you have an IPFS with a json, and in this json you have an image, you can set a link to this image that points to your server, so you could later change this, but I think in this case IPFS will loose its meaning.