**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`

});
      itRenders('no aria prop with null value', async render => {
        const e = await render(<div aria-label={null} />);
        expect(e.hasAttribute('aria-label')).toBe(false);
      });
      itRenders('"aria" attribute with a warning', async render => {
        
        const e = await render(<div aria="hello" />, 1);
        expect(e.getAttribute('aria')).toBe('hello');
      });
    });
    describe('cased attributes', function() {
      itRenders(
        'badly cased aliased HTML attribute with a warning',
        async render => {
          const e = await render(<meta httpequiv="refresh" />, 1);
          expect(e.hasAttribute('http-equiv')).toBe(false);
          expect(e.getAttribute('httpequiv')).toBe('refresh');
        },
      );
      itRenders('badly cased SVG attribute with a warning', async render => {
        const e = await render(
          <svg>
            <text textlength="10" />
          </svg>,
          1,
        );
        
        
        if (render === clientCleanRender) {