HTML5 is the latest version of HTML after the version XHTML introduced. As we know back to HTML version 4.0.1, W3c considered that HTML should be more machine readable, so they started to began introducing a new version of HTMl into a more strict markup language which is called XHTML.
XHTML was known as version 4.0.1 that was rewritten using XML format. This new document is more strict and force all the documents to be well formed and valid. Due to this restriction and the complexity to write this document, HTML5 is born. HTML5 specification is now less restrictive version of HTML. End tags are no longer required for some elements (Note: some tags still needs end tag) and the tags are no longer case sensitive and quotes for attributes values are not necessary needed.