While there’s much debate about the value of just about every on-page factor of SEO, the going knowledge is that cumulatively, all elements of your content make some difference in how your site is finally ranked, and careful attention to webmaster guidelines and accepted industry standards is key to maximizing the performance of your site… At least, the factors that you have control over.
And so, today I’d like to remark on page headings. Semantically, H1, H2, H3, etc. are all meant to structure a page, with main headings followed by subheadings, etc. Speculation is that text encapsulated by a heading tag is given special emphasis, and thus greater weight, in the search engines. This isn’t to say that you should stuff “Mortgage Cars Mortgage Real estate Broker New York” between your headings and expect top rankings, but that as a key element of design, you should have a strategy for how headings are to be integrated on your site’s pages and use them accordingly.
Many documents, for example, use a bolded paragraph tag rather than a subheading, say an <h3>, to point out the main subject of a new paragraph. In fact, many sites have their main headlines (which ought to be to-the-point and keyword dense!) as plain paragraph tags! While putting that same headline in an <h1> won’t rocket you to the top of the most competitive SERPs, it is a good standards-friendly practice, makes your page more readable without a stylesheet and heck, it can’t hurt your SEO.
Blogs especially have a long ways to go in having perfect semantic markup, explained at great length in an excellent detail at Pearsonified. In general, though CSS may make it tempting, don’t abuse unsuspecting HTML elements to get a desired effect without considering what the effects of your actions will have on your site’s overall SEO and usability.