**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`

expect(e.firstChild.getAttribute('textlength')).toBe('10');
          expect(e.firstChild.hasAttribute('textLength')).toBe(false);
        } else {
          
          
          
          expect(e.firstChild.getAttribute('textLength')).toBe('10');
          expect(e.firstChild.hasAttribute('textlength')).toBe(false);
        }
      });
      itRenders('no badly cased aliased SVG attribute alias', async render => {
        const e = await render(
          <svg>
            <text strokedasharray="10 10" />
          </svg>,
          1,
        );
        expect(e.firstChild.hasAttribute('stroke-dasharray')).toBe(false);
        expect(e.firstChild.getAttribute('strokedasharray')).toBe('10 10');
      });
      itRenders(
        'no badly cased original SVG attribute that is aliased',
        async render => {
          const e = await render(
            <svg>
              <text stroke-dasharray="10 10" />
            </svg>,
            1,