Html5 is very important for seo. It is very useful for every website. The HTML specification includes more tags for determining the layout of web documents, including <header>, <footer>, <nav> etc.
The Good things that much faster load times. A loading time is very important factor for the website quality. In this way Search engine will easliy read your content and fastly indexed your pages
The site must have a faster load times in order to have visitors to stay in a site continuously. If site takes more time to load then naturally people wont be interested to be in the site. Else go in for other sites.