A header tag is part of a web page's displayed content within the body section and contains the main title for an individual section of page content. You can have as many header tags on the page as you have sections of content. To reflect a difference in the relevant importance of, for example, the page's overall main title and the lower page section titles, there is more than one type of header tag available. Header tags are numbered from one (h1), the most important header on the page, to six (h6), the lowest. The browser will automatically format header tags, making them stand out bolder than the surrounding text, and also resizes it depending on the level of the header, and also adds an empty line underneath it. By default, the h1 header tag text is the largest text on the page. H4 text is often approximately the same font size and the main text on the page, although all header tags will be displayed as bold. If you do not like how the browser displays the header tags you can change any of the displayed features using CSS, such as font size, boldness etc., and even make it display 'inline' with other text.
Header tags are used throughout your web pages, anywhere you need to introduce a section of related text. Header tags always consist of an opening and closing tag. Do not omit the closing tag or your page will fail W3C verification tests and may not display correctly.
<h1>The main keyword rich page title at the top</h1>
<h3>A section header lower down within the content</h3>
You should always include one, but only one, single well written h1 header tag on your page. Google and the other search engines will rate the content of your h1 header as the most SEO important of your on page text. For this reason, you must always include your main keywords inside the single h1 header tag, but remembering to keep it readable by your human viewers. Provided the search engine also finds the same keywords spread throughout the text and in the title tag etc., your page will index well for those keyword terms. Don't forget, the text content gives your pages good search engine presence, and header tags must reflect this for best SEO practice. Ideally the h1 tag should be a the very start of your on page content, above everything else, as this is the place the search engine would expect to find the h1 tag. The closer it is to the start of the page content, the more important the search engines will rate this content, so this is where you need to place your main keywords. Never be tempted to include more than one h1 header tag per page in an attempt to increase the importance of your keywords. You are more likely to dilute the message. Lower down the page you should use lower header tags. If you think your page has two equally important sections that deserve their own h1 tag, you should consider splitting your page in to two separate pages so that each page can be individually SEO optimised.
In the same way you should only ever have one single h1 header tag on any web page, you should not overdo the h2 and h3 header tags. As with many aspects of advanced SEO, create a tiered structure. Keep one h1 tag per page. Limit yourself to a few h2 tags on each page, with a few h3 tags for each of the h2 tags, and so on. Make sure every single header tag on your page has some associated unique content directly beneath it. Never be tempted to contain all of the text on your page inside header tags to show to the search engines how important it all is, as they will ignore this and your message will be diluted at best and could be lost entirely. At the very least, make sure your lower h3 and h4 headers have a few sentences or a paragraph or two of content beneath them. Finally, there are no 'SEO rules' for the exact number of header tags, apart from the guideline of one h1 header tag per page. Search engines thrive on well thought out and structured unique content correctly introduced with relevant headings.
To see correctly used header tags in action, view the source code for this page!