All local files referenced by the href attribute of the <link> tag should exist.
With this small site structure:
site
| index.html
| page1.html
|
+---css
| style1.css
|
+---img
| pict1.png
|
\---js
script1.js
If index.html contains this content:
<head>
<link rel="stylesheet" href="css/style1.css">
<link rel="stylesheet" href="css/style2.css">
<!-- ... -->
</head>
The first <link> tag does not trigger any ERROR, because the file css/style1.css exists.
The second <link> tag triggers an ERROR, because there is no file css/style2.css.
Remote resources (for example starting with https://) are not checked by this rule.