**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('badly cased className with a warning', async render => {
        const e = await render(<div classname="test" />, 1);
        expect(e.hasAttribute('class')).toBe(false);
        expect(e.hasAttribute('classname')).toBe(true);
      });
      itRenders(
        'className prop when given the alias with a warning',
        async render => {
          const e = await render(<div class="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders(
        'className prop when given a badly cased alias',
        async render => {
          const e = await render(<div cLASs="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
    });
    describe('htmlFor property', function() {
      itRenders('htmlFor with string value', async render => {
        const e = await render(<div htmlFor="myFor" />);
        expect(e.getAttribute('for')).toBe('myFor');
      });
      itRenders('no badly cased htmlfor', async render => {
        const e = await render(<div htmlfor="myFor" />, 1);
        expect(e.hasAttribute('for')).toBe(false);
        expect(e.getAttribute('htmlfor')).toBe('myFor');
      });