**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`

});
  
  
  describe('custom elements', () => {
    itRenders('class for custom elements', async render => {
      const e = await render(<div is="custom-element" class="test" />, 0);
      expect(e.getAttribute('class')).toBe('test');
    });
    itRenders('className for custom elements', async render => {
      const e = await render(<div is="custom-element" className="test" />, 0);
      expect(e.getAttribute('className')).toBe('test');
    });
    itRenders('htmlFor attribute on custom elements', async render => {
      const e = await render(<div is="custom-element" htmlFor="test" />);
      expect(e.getAttribute('htmlFor')).toBe('test');
    });
    itRenders('for attribute on custom elements', async render => {
      const e = await render(<div is="custom-element" for="test" />);
      expect(e.getAttribute('for')).toBe('test');
    });
    itRenders('unknown attributes for custom elements', async render => {
      const e = await render(<custom-element foo="bar" />);
      expect(e.getAttribute('foo')).toBe('bar');
    });
    itRenders('unknown `on*` attributes for custom elements', async render => {
      const e = await render(<custom-element onunknown="bar" />);
      expect(e.getAttribute('onunknown')).toBe('bar');
    });
    itRenders('unknown boolean `true` attributes as strings', async render => {
      const e = await render(<custom-element foo={true} />);