I'm making a web browser. One of the features I'd like is to have it only show a reload button if the page has changed since it last loaded. I figured a good way of doing that would be if I could somehow request a MD5 checksum of the page every so many seconds and compare it against the checksum of the currently loaded page. Is that somehow possible? Alternatively, is there some way I could only download a portion of a page? Like, maybe my browser could just download a random portion of a page, and if it appears to be unchanged, it halts?