It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
render() {
          return <div><Child1 /><Child2 /><Child3 /><Child4 /></div>;
        }
      }
      class Child1 extends React.Component {
        render() {
          return <div class="paladin">Child1</div>;
        }
      }
      class Child2 extends React.Component {
        render() {
          return <div>Child2</div>;
        }
      }
      class Child3 extends React.Component {
        render() {
          return <div onclick="1">Child3</div>;
        }
      }
      class Child4 extends React.Component {