**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`

warning(
              false,
              'Use the `defaultValue` or `value` props instead of setting ' +
                'children on <textarea>.',
            );
          }
          invariant(
            defaultValue == null,
            'If you supply `defaultValue` on a <textarea>, do not pass children.',
          );
          if (Array.isArray(textareaChildren)) {
            invariant(
              textareaChildren.length <= 1,
              '<textarea> can only have at most one child.',
            );
            textareaChildren = textareaChildren[0];
          }
          defaultValue = '' + textareaChildren;
        }
        if (defaultValue == null) {
          defaultValue = '';
        }
        initialValue = defaultValue;
      }
      props = Object.assign({}, props, {
        value: undefined,
        children: '' + initialValue,
      });
    } else if (tag === 'select') {
      if (__DEV__) {