prefetch-purpose.php has MIME type text/html
prefetch-purpose.php has MIME type text/html
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Purpose: prefetch

This test verifies that prefetches are sent with the HTTP request header Purpose: prefetch. To do this, the root page has a prefetch link targetting this subresource which contains a PHP script (resources/prefetch-purpose.php). That initial prefetch of this resource sets a cookie. Later, the root page sets window.location to target this script, which verifies the presence of the cookie, and generates the happy test output that you hopefully see right now.