The other day I discovered js-dos, a port of DosBox to javascript! It does require a keyboard, so I don't know of a way to make it work in mobile browsers ...though I haven't tried connecting a keyboard to my phone to use it, but sounds a bit overkill.
Anyway, so what is the best use of DOS in a web browser? Well playing old DOS games, of course! So here's a throwback to my DOS programming days - my last DOS game: Pants' Escape. Tell me how far you got in the comments section! (And especially if you find the big secret...)
...In case you notice the copyright on it, yes I programmed a DOS game in 2001. But it was my last one, I promise!