**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`

return '';
    }
    
    
    let frames = currentDebugStacks[currentDebugStacks.length - 1];
    let stack = '';
    
    for (let i = frames.length - 1; i >= 0; i--) {
      const frame: Frame = frames[i];
      
      
      
      let debugElementStack = ((frame: any): FrameDev).debugElementStack;
      for (let ii = debugElementStack.length - 1; ii >= 0; ii--) {
        stack += describeStackFrame(debugElementStack[ii]);
      }
    }
    return stack;
  };
}
let didWarnDefaultInputValue = false;
let didWarnDefaultChecked = false;
let didWarnDefaultSelectValue = false;
let didWarnDefaultTextareaValue = false;
let didWarnInvalidOptionChildren = false;