**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`

);
              this.stack.push(fallbackFrame);
              out[this.suspenseDepth] += '<!--$!-->';
              
              continue;
            } else {
              out[this.suspenseDepth] += buffered;
            }
          }
          
          out[this.suspenseDepth] += footer;
          continue;
        }
        const child = frame.children[frame.childIndex++];
        let outBuffer = '';
        if (__DEV__) {
          pushCurrentDebugStack(this.stack);
          
          ((frame: any): FrameDev).debugElementStack.length = 0;
        }
        try {
          outBuffer += this.render(child, frame.context, frame.domNamespace);
        } catch (err) {
          if (enableSuspenseServerRenderer && typeof err.then === 'function') {
            suspended = true;
          } else {
            throw err;
          }
        } finally {
          if (__DEV__) {