The six heading elements, H1 through H6, denote section headings. Although the order and occurrence of headings is not constrained by the HTML DTD, documents should not skip levels (for example, from H1 to H3), as converting such documents to other representations is often problematic. Example of use:
<H1>This is a heading</H1> Here is some text
<H2>Second level heading</H2> Here is some more text.
Typical renderings are:
1) Bold, very-large font, centered. One or two blank lines above and below.
2) Bold, large font, flush-left. One or two blank lines above and below.
3) Italic, large font, slightly indented from the left margin. One or two blank lines above and below.
4) Bold, normal font, indented more than H3. One blank line above and below.
5) Italic, normal font, indented as H4. One blank line above.
6) Bold, indented same as normal text, more than H5. One blank line above.
For further information see the XHTML Quick Reference Guide.