docs: public HtmlPolicyBuilder allowUrlProtocols(String... protocols) {
      invalidateCompiledState();
      // If there is at least one allowed protocol, then allow URLs and
      // add a filter that checks href and src values.
  
      // Do not allow href and srcs through otherwise, and only allow on images
      // and links.
      for (String protocol : protocols) {
        protocol = Strings.toLowerCase(protocol);
        allowedProtocols.add(protocol);
      }
      return this;
    }