<H1> </H1>
The H stands for heading, with H1 being the most important header in the document and H6 the least. H1 is larger than any of the others, and is sometimes included in search results. Headers can be aligned to the left, center, or right.
Example:
<H1>This is header size 1.</H1>
This is header size 1.
<H2>This is header size 2.</H2>This is header size 2.
<H3>This is header size 3.</H3>This is header size 3.
<H4>This is header size 4.</H4>This is header size 4.
<H5>This is header size 5.</H5>This is header size 5.
<H6>This is header size 6.</H6>This is header size 6.
