We've got all these mechanisms built up to deal with web UI that is constructed with HTML and CSS in terms of accessibility, in terms of search indexing, in terms of browser plugins and extensions, in terms of web services and bookmarklets, in terms of UI debugging... Also, the web UI you get with HTML and CSS inherits a bunch of standard behaviors and defaults that make for more consistent experience from site to site. Consistency in UI mental models is a great thing.
I can't think of a single argument FOR this idea of rendering UI entirely in canvas that shouldn't instead be met with a response of "so lets make HTML and CSS better!" Instead of improving the open standards of HTML/CSS, people are pushing towards proprietary solutions.
Sometimes even the best intentions can go awry. I don't think this is malice so much as ignorance.