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.
it('the font-face element is not a custom element', function() {
        spyOn(console, 'error');
        var el = ReactTestUtils.renderIntoDocument(
          <svg><font-face x-height={false} /></svg>,
        );
        expect(el.querySelector('font-face').hasAttribute('x-height')).toBe(
          false,
        );
        expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'Warning: Invalid DOM property `x-height`. Did you mean `xHeight`',
        );
      });
      it('the font-face element does not allow unknown boolean values', function() {
        spyOn(console, 'error');
        var el = ReactTestUtils.renderIntoDocument(
          <svg><font-face whatever={false} /></svg>,
        );
        expect(el.querySelector('font-face').hasAttribute('whatever')).toBe(
          false,
        );
        expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'Warning: Received `false` for non-boolean attribute `whatever`.',
        );
      });
    });
  }
});