Since this is in the public thread I won't name the tools or websites used for testing, but I will discuss this with Rick over PMs.
Pure HTML sites are extremely vulnerable to mirroring as it's basically a matter of viewing the page source and copy/pasting everything, either manually or by using some tool.
I was hoping something as simple as translating everything to PHP would do the trick, but now I'm almost certain it won't.
Even if the website drew data from MySQL with PHP, all HTML output can still be mirrored.
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