Heading tags into HTML are <h1>, <h2>, <h3>, <h4>, <h5> and <h7>. This all heading tags use for font size where <h1> is for large fonts and <h6> is smallest font. These tags are used in SEO because it gives more preference to large font tags rather than small font tags for example; any keyword with <h1> includes more effectively than any other tags into the website.