I don't think the spec is 100% finalized, so maybe it's not too late to get new ideas in and implemented.
Additionally, a way to greatly speed up WWW pages is to fetch your CSS and images from more than one hostname. What I see in IE 7 is that CSS fetched from the same server as the HTML document is loaded almost serially, while that same CSS fetched from the same server but with a different hostname is loaded in parallel with the HTML. Similarly, with images, if you load 20 from one host, they get loaded 2 at a time in parallel, but if you loaded 10 from one hostname and the other 10 from another hostname, you get 4 loaded at a time in parallel.
The cost, in terms of page load speed, of all these hostnames is in the DNS lookups by the browser (first visit).
So I propose an additional tag to HTML 5. It obviously won't be supported by current browsers, but it could be implemented in all future ones.
I don't care what the actual tag name is, but for discussion I'll call it <hostname>. It's usage is fairly obvious:
When the browser sees this, it tells its DNS helper that for the rest of the document, http://cdn.sencha.com is at IP 127.0.0.1, so it doesn't have to do the DNS lookup to open a socket to that host.
<hostname name="http://cdn.sencha.com" ip="127.0.0.1" />
This would be perfectly valid:
The benefit is obvious. The server can do one DNS lookup for a host and provide that service to every client. If your page gets hit by 10,000 people in a day, the server has done the DNS lookup for 10,000 people in one swoop.
<hostname name="http://cdn.sencha.com" ip="10.0.0.1" />
<img src="http://cdn.sencha.com/a.png" />
<hostname name="http://cdn.sencha.com" ip="10.0.0.2" />
<img src="http://cdn.sencha.com/b.png" />
cdn.sencha.com doesn't do the idea justice. Imagine this scenario:
where a page might load ExtJS plugins from all over the place.
It gets even more interesting when you include a script that injects script tags directly into the DOM. As webmaster, I know what hosts those script tags reference, so I can assist the browser by telling it the DNS lookup.
Again, the DNS lookups kill performance on the browser side.