In HTML5, you can just use the following Doctype before the <html> tag.
Quote from W3Schools
The declaration must be the very first thing in your HTML5 document, before the tag.
The declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
In HTML 4.01, all declarations require a reference to a DTD, because HTML 4.01 was based on SGML.
HTML5 is not based on SGML, and therefore does not require a reference to a DTD.