I wish I had a good answer. The cache limits seem to change frequently (in a good way), and they're not well documented.
Here are some links with info:
Note that audio and video typically do not get cached because they are played by the device's media player (rather than the browser, which does the caching).
Update (May 2016):
Thanks to Chris Converse, I have confirmed that small videos can be cached offline in newer versions of iOS: