Is it not necessary to close the tag in HTML 5 like HTML? or it's a bug in W3C validator
Why this code is valid in W3C validator
<link rel="stylesheet" href="style.css">
That markup is indeed valid.
<p> tags don't have to be closed in HTML 4.01 or HTML5. I'm not sure where you got the idea that HTML5 requires everything to be closed like in XHTML.
HTML5 is just regular HTML with extra new features (hence the version jump from 4.01 to 5). It does not in any way derive from XHTML. You can close all of your HTML5 tags so it looks like well-formed XML, but the spec doesn't require you to.