The H1 tag serves as the primary heading for every web page. Specifically, H1 headings specify both users and search engines what the page constitutes. H1 headings serve an important purpose in terms of both SEO and accessibility. Impossible to overemphasise, even more, H1 serves to aid search engine interpretation of the content of the page, which in turn serves to assist with keyword rank. An H1 should thus represent the content and use keywords where exactly appropriate, but keyword stuffing must be avoided. From an accessibility perspective, screen readers will depend on H1 tags to provide structure and context for users with visual impairments. A H1 should be kept short and concise and dynamic per each page of the site. Visually, it should stand out from the rest of the headings and should be the biggest heading on the page, clearly distinguishing itself from its subheadings (H2, H3, etc.). While you could put out multiple H1s in HTML5, the best practice is to have just one per page to keep a clear semantic structure so as not to confuse search engines. Consider the H1 as being the title of a book-it should convey the topic in a clear and concise way. An interesting H1 will also positively affect click-through rates from SERPs, enticing the user to enter the page.