With nxhtml-mode you can edit XHTML documents, but not HTML dito. So what do you do with your old HTML documents? The answer is simple: You convert them to XHTML! There is today not many reasons not to convert them to XHTML. You may say "but what about old browsers?". Most users just do not have old browsers today. Old browsers are too dangerous to use on the Internet.
You can convert the documents easily from within nxhtml-mode with Tidy. However Tidy does not come with nxhtml, you have to install it yourself.
When Tidy is called from Emacs you can do a whole directory tree at once. When a buffer is in nxhtml-mode (and tidy.el is found) there is an entry on the menus called Tidy from which you can access tidy and set the options for it. Note especially the Quick Options Settings where you can set options for converting to XHTML easily.