Supporting existing content The Web celebrated its 21st birthday in 2011. Its phenomenal growth is unparalleled in history and has resulted in billions of web pages in existence already. At the heart of the development of HTML5 lies the importance of supporting all of this existing content. HTML5 isn’t about sweeping out the old and replacing it lock, stock, and barrel with something new. It's about keeping what we already have and adding enhancements on top. With this in mind, the W3C states that It should be possible to process existing HTML documents as HTML5 and get results that are compatible with the existing expectations of users and authors, based on the behavior of existing browsers.