**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`

msGridColumnStart: 10,
            }}
          />,
        );
        expect(style.lineClamp).toBe('10');
        expect(style.WebkitLineClamp).toBe('10');
        expect(style.MozFlexGrow).toBe('10');
        
        
        
        expect(style.MsFlexGrow || style.msFlexGrow).toBe('10');
        expect(style.MsGridRow || style.msGridRow).toBe('10');
        expect(style.MsGridRowEnd || style.msGridRowEnd).toBe('10');
        expect(style.MsGridRowSpan || style.msGridRowSpan).toBe('10');
        expect(style.MsGridRowStart || style.msGridRowStart).toBe('10');
        expect(style.MsGridColumn || style.msGridColumn).toBe('10');
        expect(style.MsGridColumnEnd || style.msGridColumnEnd).toBe('10');
        expect(style.MsGridColumnSpan || style.msGridColumnSpan).toBe('10');
        expect(style.MsGridColumnStart || style.msGridColumnStart).toBe('10');
      });
    });
    describe('aria attributes', function() {
      itRenders('simple strings', async render => {
        const e = await render(<div aria-label="hello" />);
        expect(e.getAttribute('aria-label')).toBe('hello');
      });
      
      itRenders('aria string prop with false value', async render => {
        const e = await render(<div aria-label={false} />);
        expect(e.getAttribute('aria-label')).toBe('false');