Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">a<!-- -->bc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".
expect(e.style.opacity).toBe('0.5');
        expect(e.style.padding).toBe('4px');
      });
      itRenders('custom properties', async render => {
        const e = await render(<div style={{'--foo': 5}} />);
        
        
        expect(e.style.Foo).toBe('5');
      });
      itRenders('no undefined styles', async render => {
        const e = await render(
          <div style={{color: undefined, width: '30px'}} />,
        );
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no null styles', async render => {
        const e = await render(<div style={{color: null, width: '30px'}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no empty styles', async render => {
        const e = await render(<div style={{color: null, width: null}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('');
        expect(e.hasAttribute('style')).toBe(false);
      });
      itRenders('unitless-number rules with prefixes', async render => {
        const {style} = await render(
          <div