Image Zoomer -- a la http://www.dynamicdrive.com/dynamicinde ... zoomer.htm

This would be interesting in some use cases;

http://s3.amazonaws.com/buildinternet/l ... deshow.htm

with FF + Webkit now representing 40% of the market in europe/USA, it would be now time to use native HTML5 APIs when available :
- drag&drop, use native when possible
- multiple file upload, with progress event and photo preview from the disk (no flash when not necessary
- drop files from desktop (fallback = java applet ... not sure for this one )
- wrapper around geolocation (fallback to IP location, configurable provider)
- OnHashChange event, fallback to a monitoring loop if it does not kill performances
- video ! for the flash player fallback, it's probably possible to use the same cssText inside the flash to style the controls. For the markup,
- leverage access to Canvas and VML for IE like (or with) http://sourceforge.net/projects/excanvas/
- offline event : not sure if it's possible (or event wishable) to ping the server to know if we are still online and trigger the event
- web workers : never tried but it might be possible to emulate this with a frame, luckily it would open a new browser thread so the heavy computing should not block the main page

that's all for HTML5, I actually think that libraries vendor like YUI have a major role to play in the adoption of the HTML5 standard (anyone for a participation to the W3C ?) in the real world, by leveraging cross browser issues and providing solid fallbacks, like you did with the marvelous YUI Storage

apart from that, in order to help developers make progressive enhancement and graceful degradation, a browser feature detector like this one : http://pastie.org/855294
The problem is that there is an almost unlimited list, here I wrote just the ones I needed for my site.

Also I'm looking for a way to detect that we are on a touchscreen device, but it might not be possible at all.