<!DOCTYPE html>
<meta charset="utf-8">
<body>
Hello, world!
If so, besides removing "Hello, world!" is there any tag that's able to be removed and it still be valid, and how do you know it's still valid?
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>