Is there a way to have an arbitrary HTML element rendered in a canvas (and then access its buffer...).
You won't get real HTML rendering to <canvas>
per se currently, because canvas context does not have functions to render HTML elements.
There are some emulations:
html2canvas project http://html2canvas.hertzen.com/index.html (basically a HTML renderer attempt built on Javascript + canvas)
HTML to SVG to <canvas>
might be possible depending on your use case:
https://github.com/miohtama/Krusovice/blob/master/src/tools/html2svg2canvas.js
Also if you are using Firefox you can hack some extended permissions and then render a DOM window to <canvas>