**Do you want to request a *feature* or report a *bug*?**

Bug

### Current Behaviour

 - `ReactDOMServer.renderToString` is mutating CSS Variable names that are inlined and in the `--camelCase` format into `--kebab-case`

### Expected Behaviour

 - `ReactDOMServer.renderToString` does not mutate the CSS Variable name, and keeps it in `--camelCase` format

### Demo

 - https://codesandbox.io/s/react-dom-server-debugging-28xsr

### Versions:

 - Issue appears in `react@16.8.6` &  `react-dom@16.8.6` 
 - Issue appears in `react@16.0.0` & `react-dom@16.0.0` 
 - (That's all I've tested so far)

### Other notes:

 - It's worth noting that `ReactDOM.render` is preserving the CSS Variable name and not mutating it. It only seems to only be an issue with `ReactDOMServer.renderToString`

return <div ref="foo" />;
          }
        }
        const e = await render(<RefComponent />);
        expect(e.getAttribute('ref')).toBe(null);
      });
      itRenders('no children attribute', async render => {
        const e = await render(React.createElement('div', {}, 'foo'));
        expect(e.getAttribute('children')).toBe(null);
      });
      itRenders('no key attribute', async render => {
        const e = await render(<div key="foo" />);
        expect(e.getAttribute('key')).toBe(null);
      });
      itRenders('no dangerouslySetInnerHTML attribute', async render => {
        const e = await render(
          <div dangerouslySetInnerHTML={{__html: '<foo />'}} />,
        );
        expect(e.getAttribute('dangerouslySetInnerHTML')).toBe(null);
      });
      itRenders('no suppressContentEditableWarning attribute', async render => {
        const e = await render(<div suppressContentEditableWarning={true} />);
        expect(e.getAttribute('suppressContentEditableWarning')).toBe(null);
      });
      itRenders('no suppressHydrationWarning attribute', async render => {
        const e = await render(<span suppressHydrationWarning={true} />);
        expect(e.getAttribute('suppressHydrationWarning')).toBe(null);
      });
    });
    describe('inline styles', function() {