An HTML tag used to identify the highest level of information on a page. In practice, page titles and headlines are usually marked with H1 tags and are the largest pieces of text on a page. H1 tags can be associated with specific styles or formatting in a web page’s CSS so that headlines always appear in a certain font, color, and size. H1s play an important role in search engine optimization (SEO) and make it easier to index content. If something is in an H1 tag, search engines know it’s important.