As more people get fed up with the poor performance and security flaws of Adobe's Flash Player, they are uninstalling the Flash plug-in from their computers. The problem is that even if a site offers up HTML5 video as an alternative to Flash, trying to get to it from Safari on a Mac will pop up the little blue LEGO block.
Mac users can now take advantage of the fact that Mobile Safari in iOS doesn't support Flash in any way, by changing the desktop user agent. A browser feature that was first widely used to get websites to serve up non-standard HTML optimized for Internet Explorer, Mac Safari 5 users can change their user agent settings by enabling the Developer menu through Preferences > Advanced. From the Developer menu, the user agent can be changed so that the browser looks like Mobile Safari 3.2.2 on the iPad, which will force sites to feed HTML5 video streams if they are available.
There are, of course, some caveats. Unlike some other browsers, changing the Safari user agent only persists for the current browser window instance. Additional browser windows or restarting the browser gets you back to the original agent. John Gruber of Daring Fireball, who originally figured all of this out, provides a Terminal command that will permanently change the user agent, but since that can break other functionality, the temporary route is recommended. For sites that only serve Flash video, users can still go Flash-free in Safari by opening the site in Google Chrome, which features its own built-in Flash player.
[via Mac OS X Hints]