11. https: everywhere

Remember this catalog?

<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
  <system systemId="https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
          uri="/share/dtds/xhtml1-strict.dtd"/>
</catalog>

Think it works with this document?

<!DOCTYPE html
  PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns:html="http://www.w3.org/1999/xhtml">
<head>
<title>Hello, world</title>
</head>
<body>
<h1>Hello, world</h1>
</body>
</html>

XML Resolver includes an option to work around this problem.