The webstorage object is very easy to use but it does only save into the users cookies. It does not get saved in the cache, so the cache can be cleared and the savegame still intact. But also at the same time, because cookies are locally stored data, the user's progress will be saved whether online or not.
If you want to access the users computer with more versatility and transparency, use the nobe-webkit plugin. You will have to right click some white space in the add plugin window and check "Show deprecated Plugins"