The only elements that are absolutely required are doctype and a non-empty title. Most minimal valid HTML doc:
<!DOCTYPE html>
<title>Foo</title> <!DOCTYPE html>
<title>Foo</title>Fun fact: you do not need to close most tags in a valid HTML5 document.
This is valid:
<!DOCTYPE html>
<title>Foo</title>
<p>paragraph
<p>second
<ul>
<li>item
</ul>
<table>
<tr><td>hi!
</table>