Run the [`fixtures/ssr` test harness](https://github.com/facebook/react/tree/master/fixtures/ssr). In the terminal, the following warning is logged:

```
Warning: Unknown prop `onClick` on <a> tag. Remove this prop from the element. For details, see https://fb.me/react-unknown-prop
  in a (at Page.js:12)
  in p (at Page.js:18)
  in div (at Page.js:17)
  in Page (at App.js:12)
  in div (at App.js:10)
  in body (at Chrome.js:17)
  in html (at Chrome.js:9)
  in Chrome (at App.js:9)
  in App (at render.js:18)
```

SSR behavior should match client-side behavior and not warn for a valid attribute.
});
    });
    describe('unknown attributes', function() {
      itRenders('no unknown attributes', async render => {
        const e = await render(<div foo="bar" />, 1);
        expect(e.getAttribute('foo')).toBe(null);
      });
      itRenders('unknown data- attributes', async render => {
        const e = await render(<div data-foo="bar" />);
        expect(e.getAttribute('data-foo')).toBe('bar');
      });
      itRenders('no unknown data- attributes with null value', async render => {
        const e = await render(<div data-foo={null} />);
        expect(e.hasAttribute('data-foo')).toBe(false);
      });
      itRenders(
        'no unknown attributes for non-standard elements',
        async render => {
          const e = await render(<nonstandard foo="bar" />, 1);
          expect(e.getAttribute('foo')).toBe(null);
        },
      );
      itRenders('unknown attributes for custom elements', async render => {
        const e = await render(<custom-element foo="bar" />);
        expect(e.getAttribute('foo')).toBe('bar');
      });
      itRenders(
        'no unknown attributes for custom elements with null value',
        async render => {
          const e = await render(<custom-element foo={null} />);