**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`

const innerHTML = props.dangerouslySetInnerHTML;
  if (innerHTML != null) {
    if (innerHTML.__html != null) {
      return innerHTML.__html;
    }
  } else {
    const content = props.children;
    if (typeof content === 'string' || typeof content === 'number') {
      return escapeTextForBrowser(content);
    }
  }
  return null;
}
function flattenTopLevelChildren(children: mixed): FlatReactChildren {
  if (!React.isValidElement(children)) {
    return toArray(children);
  }
  const element = ((children: any): ReactElement);
  if (element.type !== REACT_FRAGMENT_TYPE) {
    return [element];
  }
  const fragmentChildren = element.props.children;
  if (!React.isValidElement(fragmentChildren)) {
    return toArray(fragmentChildren);
  }
  const fragmentChildElement = ((fragmentChildren: any): ReactElement);
  return [fragmentChildElement];
}
function flattenOptionChildren(children: mixed): ?string {