Short:
* HTML apps potentially will work everywhere (if you will take care they will at least work on Firefox, Opera and Chrome).
* There are HTML5 tools and knowledge already in place. Plus years of experience in HTML can be used.
Long:
IMHO developers should look into the future here not into today but even today you can do quite a lot. XUL most probably failed because there were no tools, learning material and libraries (I'm not sure about HTA). In addition XUL worked only on single browser. I have managed to write XUL app 7 years ago but I had to invest quite a lot of time and the whole development process was quite slow.
HTML apps meanwhile do not require quite a lot of investment and if you already have some HTML5/CSS/JS knowledge there are only several holes of knowledge to fill. The only problem is to select good tools, find out right information and you can start right now. E.g. if you have canvas based game or static web site with useful information you can create app now: just add application cache's manifest file, add some meta information (in case of Firefox OS create *.webapp file) and you are done.
My background: I don't know HTA. I am familiar with XUL. In addition I'm familiar with QtQuick (QML + JavaScript), Firefox OS Apps, I guess I could learn developing Chrome Apps pretty quickly. I'm familiar with native development for Windows, Linux Desktop, Android and iOS.