According to Apple, Safari is the only HTML5 Compliant Browser

that is, demos are not available if you use chrome or firefox.

  You’ll need to download Safari to view this demo.
  This demo was designed with the latest web standards
  supported by Safari. If you’d like to experience this demo, 
  simply download Safari. It’s free for Mac and PC, and it 
  only takes a few minutes.

Requiring Safari 4: How is that any different than requiring a plugin, unless you just happen to already use Safari?