Google Blog: Updating our technical Webmaster Guidelines
Updated advice for optimal indexing
Historically, Google indexing systems resembled old text-only browsers, such as Lynx, and that’s what our Webmaster Guidelines said. Now, with indexing based on page rendering, it’s no longer accurate to see our indexing systems as a text-only browser. Instead, a more accurate approximation is a modern web browser. With that new perspective, keep the following in mind:
Just like modern browsers, our rendering engine might not support all of the technologies a page uses. Make sure your web design adheres to the principles of progressive enhancement as this helps our systems (and a wider range of browsers) see usable content and basic functionality when certain web design features are not yet supported.
Pages that render quickly not only help users get to your content easier, but make indexing of those pages more efficient too. We advise you follow the best practices for page performance optimization, specifically:
Eliminate unnecessary downloads