**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`

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    }
    if (__DEV__) {
      validatePropertiesInDevelopment(tag, props);
    }
    assertValidProps(tag, props);
    let out = createOpenTagMarkup(
      element.type,
      tag,
      props,
      namespace,
      this.makeStaticMarkup,
      this.stack.length === 1,
    );
    let footer = '';
    if (omittedCloseTags.hasOwnProperty(tag)) {
      out += '/>';
    } else {
      out += '>';
      footer = '</' + element.type + '>';
    }
    let children;
    const innerMarkup = getNonChildrenInnerMarkup(props);
    if (innerMarkup != null) {
      children = [];
      if (newlineEatingTags[tag] && innerMarkup.charAt(0) === '\n') {