**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 fallbackChildren = toArray(fallback);
            const nextChildren = toArray(
              ((nextChild: any): ReactElement).props.children,
            );
            const fallbackFrame: Frame = {
              type: null,
              domNamespace: parentNamespace,
              children: fallbackChildren,
              childIndex: 0,
              context: context,
              footer: '<!--/$-->',
            };
            const frame: Frame = {
              fallbackFrame,
              type: REACT_SUSPENSE_TYPE,
              domNamespace: parentNamespace,
              children: nextChildren,
              childIndex: 0,
              context: context,
              footer: '<!--/$-->',
            };
            if (__DEV__) {
              ((frame: any): FrameDev).debugElementStack = [];
              ((fallbackFrame: any): FrameDev).debugElementStack = [];
            }
            this.stack.push(frame);
            this.suspenseDepth++;
            return '<!--$-->';
          } else {
            invariant(false, 'ReactDOMServer does not yet support Suspense.');