data:text/html, <html contenteditable>
If you then do a Save As you can save the resulting HTML page with content to your file system, and if you open it again in the browser, you can start editing it again (at least all that works in Chrome). A friend has found that Ctrl-B and Ctrl-I in the edit window work for bold and italics, and I then figured out Ctrl-U works for underlining, too. I thought that was pretty cool, and put the above as a bookmarklet link in my bookmarks bar.
Then, when talking about it with some friends, I decided to take it a bit further and created a bookmarklet that starts by copying over text you've highlighted on another web page (right now it supports just the text, not any markup) and opening the new editable window with that text in it (if you haven't highlighted anything, it just opens a blank, editable page). Here it is for your perusal. I have it as a bookmarklet in my bookmarks bar, and it works for me in Chrome.
Note: That all needs to be on one line when you make the bookmarklet, hopefully it will copy and paste that way, but you should be able to just drag the link to your bookmarks bar to save it there.