# 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I would like to close those PRs as outdated, and reboot the discussion about this.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

render() {
          return <input>moo</input>;
        }
      }
      const container = document.createElement('div');
      let caughtErr;
      try {
        ReactDOM.render(<X />, container);
      } catch (err) {
        caughtErr = err;
      }
      expect(caughtErr).not.toBe(undefined);
      expect(normalizeCodeLocInfo(caughtErr.message)).toContain(
        'input is a void element tag and must neither have `children` ' +
          'nor use `dangerouslySetInnerHTML`.' +
          (__DEV__ ? '\n    in input (at **)' + '\n    in X (at **)' : ''),
      );
    });
    it('should support custom elements which extend native elements', () => {
      const container = document.createElement('div');
      spyOnDevAndProd(document, 'createElement').and.callThrough();
      ReactDOM.render(<div is="custom-div" />, container);
      expect(document.createElement).toHaveBeenCalledWith('div', {
        is: 'custom-div',
      });
    });
    it('should work load and error events on <image> element in SVG', () => {
      spyOnDevAndProd(console, 'log');
      const container = document.createElement('div');