The guide to create your Web site on Internet

<h1> <h2> <h3> <h4> <h5> <h6> : heading
A heading element briefly describes the topic of the section it introduces. Heading information may be used by user agents, for example, to construct a table of contents for a document automatically.
There are six levels of headings in HTML with H1 as the most important and H6 as the least. Visual browsers usually render more important headings in larger fonts than less important ones.

Example

<h1> the text </h1>
<h2> the text </h2>
<h3> the text </h3>
<h4> the text </h4>
<h5> the text </h5>
<h6> the text </h6>

Gives

the text

For complete reference see html reference at W3C
PC..19 PC..20 PD..1 PD..2 PD..3 PD..4 PD..5 PD..6 PD..7 PD..8 PD..9 PD..10 PD..11 PD..12
PD..13 PD..14 PD..15 PD..16 PD..17 PD..18 PD..19 PD..20 PE..1 PE..2 PE..3 PE..4 PE..5 PE..6
PE..7 PF..1 PF..2 PG..1 PG..2 PH..1 PH..2 PH..3 PH..4 PH..5 PI..1 PI..2 PI..3 PI..4
PI..5 PI..6 PI..7 PI..8 PI..9 PI..10 PI..11 PI..12 PI..13 PI..14 PI..15 PI..16 PI..17 PI..18