This is part of the common header.

This page is in the subfolder.

Since the SSI header is pulled into this context, the resources the relative URLs in it point to are no longer located by this page since they are one directory up (../) from the subfolder.

In general, a relative URL can't be accurate from two different locations.

The solution to this problem is outlined in the source code of the site_header.php file.

This is part of the common footer.