# This "good first bug" is taken by @YongPilMoon. Don't work on it unless that's you!

This has been discussed a few times before but I don’t think there was any conclusion, and PRs intending to solve the issue were not merged for various reasons.

I would like to close those PRs as outdated, and reboot the discussion about this.
Performance considered, the conclusion from @sebmarkbage and @syranide seems to be:

> We should consider not allowing conflicting style rules at all.

— https://github.com/facebook/react/pull/2013#issuecomment-57338177

> IMHO, all things considered it's better to just disallow overlapping and warn in dev.

— https://github.com/facebook/react/pull/4661#issuecomment-132996649

Radium by @ianobermiller came to the same conclusion in https://github.com/FormidableLabs/radium/issues/95 but there was some backlash afterwards. React Native seems to allow style expansions but only for a few attributes (e.g. `margin` and `padding`, but not `border`).

I’m closing old pull requests about this, and creating this issue to track implementation of the behavior we seem to agree upon: we should warn in `__DEV__` when `border` and `borderBottom` are used at the same time. We can discuss more specifics (should either be ignored? should we allow a few whitelisted properties but warn for others?) in this issue.

As a migration strategy, we can suggest people to use something like https://github.com/ActionIQ/style-builder if they really need those shortcuts. It’s also something we’ll need to decide upon when implementing integrated styling.

---

Related issues:
- #2231
- #2407
- #5030
- https://github.com/FormidableLabs/radium/issues/95

Related PRs:
- #2013
- #4661

beforeEach(() => {
      mountComponent = function(props) {
        const container = document.createElement('div');
        ReactDOM.render(<div {...props} />, container);
      };
    });
    it('should work error event on <source> element', () => {
      spyOnDevAndProd(console, 'log');
      const container = document.createElement('div');
      ReactDOM.render(
        <video>
          <source
            src="http:example.org/video"
            type="video/mp4"
            onError={e => console.log('onError called')}
          />
        </video>,
        container,
      );
      const errorEvent = document.createEvent('Event');
      errorEvent.initEvent('error', false, false);
      container.getElementsByTagName('source')[0].dispatchEvent(errorEvent);
      if (__DEV__) {
        expect(console.log).toHaveBeenCalledTimes(1);
        expect(console.log.calls.argsFor(0)[0]).toContain('onError called');
      }
    });
    it('should not duplicate uppercased selfclosing tags', () => {
      class Container extends React.Component {
        render() {