An h1–h6 element must have both a start tag and an end tag.1
h1–h6 elements are block level elements by default (CSS style: display: block).2
h1–h6 elements should not be confused with the section element
Heading tags (h1–h6) are not related to the head tag.
Permitted Content: phrasing content
The different CSS-styles for headings differ usually in font-size and margin. The following CSS-settings for h1–h6 elements can serve as an orientation (characterized as 'informative' by the W3C)
Search engine spiders (the code that adds a page to a search engine) automatically pays more attention to higher importance (h1 has most, h2 has less, h3 has even less, ...) headings to discern what a page is about.
Headings can be used to describe the topic they precede and they are defined with the <h1> to <h6> tags. Headings support all the global attributes.
<h1> defines the most important heading.<h6> defines the least important heading.Defining a heading:
<h1>Heading 1</h1>
<h2>Heading 2</h2>
<h3>Heading 3</h3>
<h4>Heading 4</h4>
<h5>Heading 5</h5>
<h6>Heading 6</h6>
Search engines and other user agents usually index page content based on heading elements, for example to create a table of contents, so using the correct structure for headings is important.
In general, an article should have one h1 element for the main title followed by h2 subtitles – going down a layer if necessary. If there are h1 elements on a higher level they shoudn't be used to describe any lower level content.
Example document (extra intendation to illustrate hierarchy):
<h1>Main title</h1>
<p>Introduction</p>
<h2>Reasons</h2>
<h3>Reason 1</h3>
<p>Paragraph</p>
<h3>Reason 2</h3>
<p>Paragraph</p>
<h2>In conclusion</h2>
<p>Paragraph</p>
<h1>...</h1><h2>...</h2><h3>...</h3><h4>...</h4><h5>...</h5><h6>...</h6>