I'm only operating my offline boardgame site through IE9 (my coding skills aren't altogether there with regards to multiple-broweser support).
If I am correct, web storage, if that's the same as 'data storage' (and I will investigate this in a minute), is NOT a working component of IE9... yet.
As for php, I took a small course on php over 10 years ago, and even then struggled with it a bit. If I remember correctly, I can use MySQL to store data on my own computer, which would be nice. But... if I send out copies of my 'OFFLINE' site to friends so that they can help me playtest, they would ALSO require MySQL on their own computers, I believe. Which wouldn't be cool.
Again, I would love a save ability for this boardgame playtesting site, so any help would be absolutely sweet (kinda like candy, only sweeter).