As browsers have standardised, it has become possible to dynamically style HTML, including making chunks of page appear and disappear. It gets more modern and interesting after the break.
Combining parsing and styling, you get syntax highlighting, either the basic Prettify hosted by Google or the Dream Productions Syntax Highlighter. You can cut out CSS altogether and style your HTML with JSS
Via the AFLAX library you can play, pause and rewind Flash videos (so long as users have the Flash 8 plug-in).