**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`

'named imports.';
        }
        const ownerName = owner ? getComponentName(owner) : null;
        if (ownerName) {
          info += '\n\nCheck the render method of `' + ownerName + '`.';
        }
      }
      invariant(
        false,
        'Element type is invalid: expected a string (for built-in ' +
          'components) or a class/function (for composite components) ' +
          'but got: %s.%s',
        elementType == null ? elementType : typeof elementType,
        info,
      );
    }
  }
  renderDOM(
    element: ReactElement,
    context: Object,
    parentNamespace: string,
  ): string {
    const tag = element.type.toLowerCase();
    let namespace = parentNamespace;
    if (parentNamespace === Namespaces.html) {
      namespace = getIntrinsicNamespace(tag);
    }
    if (__DEV__) {
      if (namespace === Namespaces.html) {