"Rendering via DOM" enables rich text via HTML which is much more (and nicer) than "bold and italic fonts" and "images". For example look at the 'domterm help' output in the right upper pane in http://domterm.org/images/domterm-panes-1.png - it is not true than many terminals can display this. (Except of course in the trivial sense of using a library to render to an image and then displaying the image.)

It is also worth pointing out that most of the features you mention (except 1 and maybe 3) use protocols that are not widely supported. DomTerm uses familiar HTML wrapped in a trivial escape sequence. A related benefit is that reflow on window resize works - and you can copy/paste or save as HTML.

"Awful slow" is relative. DomTerm does take some extra seconds if you 'cat' a large file - but if that is your primary measure of a terminal emulator then our needs are very different.