HTML5 DOCTYPE is designed as simplify version of HTML4 DOCTYPE. You can simple declare it via the following element tag. No need to spell out HTML version and DTD schema for the HTML5 pages.
<!DOCTYPE html>
DOCTYPE is normally used to instruct browser of the mode it needs to use for the web page rendering. There are several modes that were available in HTML4 and prior versions such as Quirks, Standard and Almost Standard. HTML5 DOCTYPE renders everything in Standard mode. This causes to fire page validation if HTML elements are outdated or depreciated
Character set has also been simplified and you can declare it via the following element tag.
<meta charset="utf-8">