The earliest specifications for HTML included provisions for document headings and subheadings, elements known as heading tags. Heading tag elements begin at h1 and progress to h6, each level is intended to represent an ordered and organized taxonomy. These tags serve multiple roles.
As mark-up tags, these elements conveniently format the text elements to which they are applied. For example, an h1 tag will generate large text, in a bold font, with margins above and below—much like the headline in a newspaper article.
As HTML standards matured and CSS formatting became available in browsers, many web designers abandoned heading tags in search of prettier formatting for their headings. True, you can make text big and bold with CSS mark-up or with HTML formatting; you can make any text look like a heading without utilizing heading tags. However, in the world of search that is a blunder because you will build almost no search engine ranking power from simply formatting text.
For search engines, heading tags serve a supplemental function beyond formatting. Heading tags serve as signposts, that help search engines determine the context and topic of a web page. Heading tags are certainly part of search algorithms, and are given moderate weight in determining search position.
The best methodology for employing heading tags is to do the following. First, your page should only employ an h1 tag once. The text of the h1 tag should describe the main topic of the web page upon which it appears and should include the high-value, high-volume keywords for which you want to rank. Next, at a minimum you should employ both h1 and h2 tags. Your h2 tags should repeat your important keywords—but with additional terms to give context to the section that the h2 tag covers. For example, if you are writing a page about air conditioning service and repair, you might employ h2 tags with text such as, Your best choice for air conditioning service, and Licensed and insured air conditioning service.
The h3 tags are optional, but can come in handy for organizing longer pages. The search engine ranking power of heading tags decreases as you progress from h1 down through the lower orders of heading tags. Thus, h1 tags are mandatory, h2 tags are highly recommended, and h3 tags are necessary only in the most competitive markets. Heading tags have a complementary effect when combined with an effective title tag, body text, and meta description—when these elements are in accord, a search engine can more comfortably determine the main context of a web page, and can more confidently reward that web page with higher rankings.
Remember also, that keyword prominence applies to heading tags, so greater weight is given to the words at the beginning of the tag.