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.
expect(spanNode.tagName).toBe('SPAN');
        expect(spanNode.childNodes.length).toBe(1);
        expectNode(spanNode.firstChild, TEXT_NODE_TYPE, 'More Text');
      });
      itRenders('a non-standard element with text', async render => {
        const e = await render(<nonstandard>Text</nonstandard>);
        expect(e.tagName).toBe('NONSTANDARD');
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, 'Text');
      });
      itRenders('a custom element with text', async render => {
        const e = await render(<custom-element>Text</custom-element>);
        expect(e.tagName).toBe('CUSTOM-ELEMENT');
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, 'Text');
      });
      itRenders('a leading blank child with a text sibling', async render => {
        const e = await render(<div>{''}foo</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          if (render === serverRender || render === streamRender) {
            expect(e.childNodes.length).toBe(1);
            expectTextNode(e.childNodes[0], 'foo');
          } else {
            expect(e.childNodes.length).toBe(2);
            expectTextNode(e.childNodes[0], '');
            expectTextNode(e.childNodes[1], 'foo');
          }
        } else {