Headline tags (hx tags)

<h1> - <h6>: Headline tags
<h1>: Most important headline, you must have every page, and only one <h1> tag in a page
<h2>: Subheading, used for sections within your main content
<h3> - <h6>: Used for further subdivisions of content
You should use <h1> to <h6> tags in order

REASON

SEO: Search engines use headline tags to understand the structure and content of your page
Accessibility: Screen readers use headline tags to help visually impaired users navigate your content
Readability: Headline tags help users quickly scan your content and understand the hierarchy of information

Example

BAD

You must include <h1> tag in every page

<body>
    <section>
        <h2>Subheading</h2>
        <h3>Sub-subheading</h3>
    </section>
</body>

You should not skip heading levels

<body>
    <h1>Heading Title</h1>

    <section>
        <h3>Sub-subheading</h3>
    </section>

    <section>
        <h2>Subheading</h2>
        <h3>Sub-subheading</h3>
    </section>
</body>

GOOD

<body>
    <h1>Heading Title</h1>

    <section>
        <h2>Subheading</h2>
        <h3>Sub-subheading</h3>
    </section>

    <section>
        <h2>Subheading</h2>
        <h3>Sub-subheading</h3>
    </section>
</body>

Example​

Example