I've just been reading the HTML5 author spec.
It states that the <html>, <head> and <body> tags are optional.
Does that mean that you can leave them out completely and still have a valid HTML5 document?
If I'm interpreting this correctly, it means this should be completely valid:
<!DOCTYPE html>
<p>Hello!</p>
Is this correct?
You can check out the spec here:
http://dev.w3.org/html5/spec-author-view/syntax.html#syntax
"8.1.2.4 Optional tags" is the bit out about it being OK to omit <html>, <head> and <body>
This is the minimal HTML5-valid document:
<!doctype html><title> </title>
The title element is indeed required, but as Jukka Korpela notes, it also must be non-empty. Furthermore, the content model of the title element is:
Text that is not inter-element white space.
Therefore, having just a space character in the title element is not considered valid HTML. You can check this in W3C validator.
So, an example of a minimal and valid HTML5 document is the following:
<!doctype html><title>a</title>
W3C HTML validator maintainer here. FYI with regard to the validator behavior, as of today, the validator now enforces the requirement in the HTML spec that the title element must contain at least one non-whitespace character -
While the <html>, <head> and <body> start and end tags are optional, the <title> tags are required, except in special circumstances, so no, your sample is not (ordinarily) valid.
I think you are reading it correctly. Although browsers will even render incorrect HTML (try breaking the rules and FF will render the same).
来源：https://stackoverflow.com/questions/9797046/whats-a-valid-html5-document