The <!DOCTYPE> declaration must be the very first thing in your HTML document, before the <html> tag.
The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
其实简单来讲，就是告诉浏览器使用那个HTML版本进行解释，千万不要忘记在最开始的时候写<!DOCTYPE html> 。
这个是HTML５的，HTML４的跟这个不一样，其实要用的时候抄过来就行。