HTML5 needs to have declaration as first thing in your HTML5 document. Users browser uses this tag to determine which HTML specification your document uses. The doctype in HTML 4.01 requires reference to DTD, because HTML 4.01 was based on SGML. HTML5 is not based on SGML, and does not require a reference to a DTD, but needs a doctype for browsers to tell them how to behave.
There is the one simple line you should put on top of your HTML document to make it HTML5. Copy the following snippet and paste it to your new HTML to use HTML5 doctype: