Although I say SEO templates, SEF templates would be a good term as well. W3C compliant templates are focused on XHTML (eXtensible Hyper Text Markup Language) and CSS Cascading Style Sheets, which means the markup of the site's template is split into generating the HTML code, and the real layout is done by a CSS file.
Templates you use for your web site should follow the W3C coding as closely as possible. Search Engine Robots are built to look for proper coding and get their impressions from that. This means you want your page titles enclosed in H1 tags (meaning this is the most important line on this page), subheadings in H2, and so on. Subheadings in H2 can be used on the site for multiple headings, but H1 tags should appear only once.
Some templates form the site title in a header like text and post it as H1, this is of course not the most optimal coding decision but it is very common in Joomla! templates. If you want to check the used elements of a...