So, I thought I would produce a test that drew a graphical representation of dt with respect to time.
Try this demo. I didn't get the results I was expecting, for sure.... A new sprite dot is drawn every tick and its y value depends on dt. My min/max values, after a few seconds to let the browsers settle down, were:
Chrome: 15 - 18 ms,
Chrome Canary 15 - 18 ms (visually slightly smoother)
Firefox 15 - 21 ms (relatively poor performance)
IE 16.7 +/- 0.2 (appeared to be the only browser that tries to target the vsync dt)
There is clearly a different philosophy between IE and the other browsers - IE clearly tries to target vsync whereas the others seem to hit slightly above or slightly below the target. Let me know what you think, but it doesn't look like Canary Chrome has sorted itself out yet....