docs: public HtmlPolicyBuilder allowElements(
        ElementPolicy policy, String... elementNames) {
      invalidateCompiledState();
      for (String elementName : elementNames) {
        elementName = HtmlLexer.canonicalName(elementName);
        ElementPolicy newPolicy = ElementPolicy.Util.join(
            elPolicies.get(elementName), policy);
        // Don't remove if newPolicy is the always reject policy since we want
        // that to infect later allowElement calls for this particular element
        // name.  rejects should have higher priority than allows.
        elPolicies.put(elementName, newPolicy);
        if (!textContainers.containsKey(elementName)) {
          if (METADATA.canContainPlainText(METADATA.indexForName(elementName))) {
            textContainers.put(elementName, true);
          }
        }
      }
      return this;
    }