This has a very annoying (and medal-breaking) loading bug. It seems somewhere starting around level 5 or level 6 (maybe sooner maybe later) after completing any level it just hangs on that loading screen for the next and you'll need to refresh browser and manually select the next level from level select. And once this starts it consistently happens for every level since.
The problem is this resets all your lives (any extras are lost) and apparently counts as you having died ("No Deaths" medal becomes impossible to obtain).
Also I notice 10 Secret Boxes medal is impossible to obtain as well, probably because it needs to be 10 boxes in one game, and reloading the page counts as another game? >:(
And the 10 levels faster than default I also assume must be done in one game which, again, isn't possible due to this between-stages loading bug.
That's three high-tier medals that don't seem obtainable due to a single game bug!
Tried on both Firefox and Chrome. Same result.