I have a good news for everyone who still uses a PPC based Mac running Mac OS X 10.5 and want to use a modern browser. WebKit nightly builds (available from http://www.webkit.org/) only support Intel and Mac OS X 10.6 or higher. But I found a project which is called Leopard WebKit making these builds to run under PPC. (available from http://code.google.com/p/leopard-webkit/) It's still just Safari 5.0.6, but with a modern WebKit. I can't guarantee that it works.