It's a relative path because you're not listing the full link (like it would appear in your browser's address bar). What I typically do is create a folder in the same "directory" that the home page appears called "styles" (I do the same for images), and then reference it by adding a forward slash, like so:
HTML Code:
<link rel="stylesheet" type="text/css" href="/styles/screen.css">
That way, no matter where the page is on the server, it'll still look to the "root" for the "styles" folder and then look for the style sheet inside it.
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