HTML has been in continuous evolution since it was introduced to the Internet in the early 1990’s. Some features were introduced in specifications; others were introduced in software releases. In some respects, implementations and author practices have converged with each other and with specifications and standards, but in other ways, they continue to diverge.
This document describes the differences between HTML4 and HTML5 and provides some of the rationale for the changes. This document may not provide accurate information as the HTML5 specification is still in development. When in doubt, always check the HTML5 specification itself.