It's not valid. To check it you can run it in W3C Validator
The error is: Element head is missing a required instance of child element title.
...
UPDATE
As vcsjones stated the head
element is optional. That's the title
one is required. Credit to mootinator for pointing out that the body
is also optional.
So the simplest valid document will be:
<!DOCTYPE html>
<title></title>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…