The key to optimizing HTML for Contiki is to pare things down to the bare minimum. Remember that Contiki is most likely running on an extremely resource-constrained system, it's up to you to meet it on its own terms.
... Respect the W3C...
The good news is that you don't need to resort to non-standard HTML on account of Contiki. Although it's true that Contiki has no use for the "DOCTYPE" and "HEAD" tags necessary to keep the W3C happy, it's also equally true that, kept to a bare minimum, they won't have any noticeable adverse effect on Contiki's performance. So *please* keep it valid for all our sakes!
... Get to the Point!
One of the obvious limitations of Contiki is the lack of available screen space -- we're talking 78 x 16 on an enhanced Apple IIe, and about half that in the 40-column version! Bear in mind that Contiki has to reload the entire page from the server every time you scroll down (and scrolling up is not yet implemented). Add to this the fact that Contiki doesn't handle "anchor" tags properly, and it quickly becomes apparent that you have to keep your important content front and center. Putting Google ads at the top of every page just doesn't cut it with Contiki!