Inside the BODY element, heading elements H1 through H6 are generally used for major divisions of the document. Headings are not mandatory. Headings are permitted to appear in any order, but you will obtain the best results when your documents are displayed in a browser if you follow these guidelines:
H1: should be used as the highest level of heading, H2 as the next highest, and so forth.
You should not skip heading levels: e.g., an H3 should not appear after an H1, unless there is an H2 between them.
The size of the text surrounded by a heading element varies from very large in an <H1> tag to very small in an <H6> tag.
This are the headings of 1 to 6:
Heading 1
Heading 2
Heading 3
Heading 4
Heading 5
Heading 6
This is the source:
<H1>Heading 1</H1>
<H2>Heading 2</H2>
<H3>Heading 3</H3>
<H4>Heading 4</H4>
<H5>Heading 5</H5>
<H6>Heading 6</H6>
