The DOM/CSS/JS is a crappy, low-level, do-everything-yourself model for creating applications when compared to the state of the art in native UI/application frameworks. The result is a hodgepodge mishmash of broken, inconsistent application UIs.
Couple that with the inability for any existing browsers' JS runtime to make use of SMP in a single app (despite devices adopting multicore ARM), limited uptake of WebGL, no support for dropping to NEON or even just C (you pay for inefficiency with battery life and poor user experience), etc.
I'd rather see Google produce a NaCL-based set of application libraries that can run on both their Android phones, ChromeOS devices, and our existing desktops. Mozilla's insistence in continuing to invest in pure HTML/JS/CSS is myopic.