The HTML Canvas element is a work in progress that isn't really ready in many cases. I've started a running list of the busted things that I have found so far with links to more information about them and fixes if I have them.

gistfile1.md

Text Kerning/Letter Spacing is missing : [https://gist.github.com/talltyler/5345894]

TextMetrics needs height : [https://gist.github.com/talltyler/5345959]

Filters Shadows have been broken in Chrome since version 15 (https://code.google.com/p/chromium/issues/detail?id=100703)

Inner shadows are missing so you have to do all of this to reproduce it http://jsfiddle.net/cabanier/eav5V/

Bitmaps For loops over every pixel are slow in many cases so most types of raw pixel modifications like replacements for the missing filters can't be used. These are the best that I have : found https://github.com/mozilla/shumway/blob/master/src/swf/filters/filter.js http://peternitsch.net/bitmapdata.js/colorMatrixFilter.html

Pixel shaders or a way to run fast matrix math would speed up many things, you can do some of this in webgl but IE wont support it http://evanw.github.io/glfx.js/demo/

Hit tests, without this every game engine has created a custom one, this has been proposed but not implemented. http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#hit-regions

General things that might help adoption Scene graph with events and hit detection http://www.createjs.com https://code.google.com/p/cakejs/