HTML5 is a markup language for structuring and presenting content for the World Wide Web. It is the fifth version of the HTML standard. HTML5 improved usability and user experience for websites, especially those with lots of audio and video as it can replace Flash and Silverlight, which are not SEO-friendly.

HTLM5 also impacts SEO for pages with large articles as it contains the <article>, <section> and <header> tags. This means HTML5 pages can have multiple H1 tags without damaging their SEO.