Like (all?) other browsers, the Kindle browser allows opening an HTML file from the local file system using 'file://..' syntax. I have confirmed this by opening a file I found in the file listing on Igor S.'s 'Reversing Everything' blog. Thus if you type this in the address field:
file:///opt/amazon/ebook/prefs/NowNowTemplate.html
It loads up.
However, I have not been able to discover the system path to the root of the user documents folder (with Audible/documents/music/system folders etc.), which you see with Explorer/Finder when the Kindle is tethered via USB.
Does anybody know the correct file path to use to get there? I think it would be useful to be able to access HTML files stored on Kindle as an alternative to converting them to AZW. For one thing, they can have JavaScript in them...
Found my answer on another thread here. The root is /mnt/us/, so if you create a folder named 'html' with a file named 'test.html', you can load it by typing this: 'file:///mnt/us/html/test.html'.
Apparently this doesn't work on K1 but I can't confirm this; I would think it would work (possibly using a different root).
If you just rename the file to xxx.txt instead of xxx.html, and put it in the documents directory, it will show up like any book, and be opened just fine. See the thread http://www.mobileread.com/forums/showthread.php?t=24127.