How to include a file based .css stylesheet

<head> <link REL="stylesheet" HREF="/mystylesheetfile.css" TYPE="text/css"> <script type="text/javascript">var switchTo5x=true;</script> <script type="text/javascript" src="https://ws.sharethis.com/button/buttons.js"></script> <script type="text/javascript">stLight.options({publisher: "83cf11a5-04b0-47d5-b5d0-fecf32207997", doNotHash: false, doNotCopy: false, hashAddressBar: false});</script> </head> ... <body>