**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 expanded = {};
  for (const key in styles) {
    const longhands = shorthandToLonghand[key] || [key];
    for (let i = 0; i < longhands.length; i++) {
      expanded[longhands[i]] = key;
    }
  }
  return expanded;
}
/**
 * When mixing shorthand and longhand property names, we warn during updates if
 * we expect an incorrect result to occur. In particular, we warn for:
 *
 * Updating a shorthand property (longhand gets overwritten):
 *   {font: 'foo', fontVariant: 'bar'} -> {font: 'baz', fontVariant: 'bar'}
 *   becomes .style.font = 'baz'
 * Removing a shorthand property (longhand gets lost too):
 *   {font: 'foo', fontVariant: 'bar'} -> {fontVariant: 'bar'}
 *   becomes .style.font = ''
 * Removing a longhand property (should revert to shorthand; doesn't):
 *   {font: 'foo', fontVariant: 'bar'} -> {font: 'foo'}
 *   becomes .style.fontVariant = ''
 */
export function validateShorthandPropertyCollisionInDev(
  styleUpdates,
  nextStyles,
) {
  if (!warnAboutShorthandPropertyCollision) {
    return;
  }