Nifty little trick we're using in Land of Seylia at the moment: hijack canvas methods to compress our save files as a PNG :). Here's the raw canvas-based compression code, in case anyone else wants to use it (or play with images). License: as free as you want - we're using CC0, which is intended as a very sticky declaration of release into public domain.
Smart move.
Probably one of the cutest savegames I've ever seen.
:)
PS: I can almost perceive the entropy from here.
*compression joke*
For anyone who's interested, we also uploaded this code to GitHub after someone suggested doing so. Find it here: Github.com