Entire website in a single HTML file
This is something I've always wanted for virtual textbooks. A single .html file with all the JS, CSS, images as inline data blobs, etc. For most physics, math, CS, etc books, this should be possible. It would make it trivial to download the book for offline use and share it with others. Other than the raster images, the entire content of these types of books would probably fit in a couple MB of text. The use of #anchors or other smart URL manipulation would also make it easy to share internal hyperlinks if you want to tell a friend to look at chapter/section #foo_bar of the book.