I wrote last year a post about content separation that suggested a way to separate the main content of a page from other content that's not very interesting. Most of the elements of a template (navigation, footer etc.) could confuse search engines into thinking a page talks about something else than it does. As a result, a page could end up ranking well for unrelated queries and not so well for the right queries.
As a solution for this problem, Yahoo introduces a 'robots-nocontent' class that can be added to any HTML tag.
"This tag is really about our crawler focusing on the main content of your page and targeting the right pages on your site for specific search queries. Since a particular source is limited to the number of times it appears in the top ten, it's important that the proper matching and targeting occur in order to increase both the traffic as well as the conversion on your site. It also improves the abstracts for your pages in results by omitting unrelated text from search result summaries.
To do this, webmasters can now mark parts of a page with a 'robots-nocontent' tag which will indicate to our crawler what parts of a page are unrelated to the main content and are only useful for visitors. We won't use the terms contained in these special tagged sections as information for finding the page or for the abstract in the search results."
While this could be useful to reduce the importance of unrelated parts of your site (like AdSense's section targeting), I can't stop wondering if this isn't the search engine's job. For example, Google can detect the navigation links from a page (you can notice this if you use the mobile version), but I don't think it minimizes the importance of the keywords used in that area.
Yahoo Lets You Delimit Unimportant Content From a Page
