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W3C Validator - HTML [validator.w3.org]
W3C Validator - CSS [jigsaw.w3.org]
I am using an external style sheet so my site is sitename.shtml
Maybe I misunderstand your situation, but you can use either of these approaches in the <head> section
<link rel="stylesheet" type="text/css" href="/path/to/styles.css" />
<style type="text/css"> @import url(path/to/styles.css); </style>
...to bring in an external style sheet - no need for SSI.