Trying to validate my site, but keep running into a series of errors I don't understand. They all are some variation of this:
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Warning Line 124, Column 34: cannot generate system identifier for general entity "id".

<a href="index.php?main_page=page&id=9&chapter=0">
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It then underlines the id part right our of the link URL. It's parsing the link strings? How can I avoid/fix this?

Thanks!