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I would like to close those PRs as outdated, and reboot the discussion about this.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

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

Related PRs:
- #2013
- #4661

const setup = {
        display: 'block',
        left: '1px',
        top: 2,
        fontFamily: 'Arial',
      };
      ReactDOM.render(<div style={setup} />, container);
      expect(stubStyle.display).toEqual('block');
      expect(stubStyle.left).toEqual('1px');
      expect(stubStyle.top).toEqual('2px');
      expect(stubStyle.fontFamily).toEqual('Arial');
      
      const reset = {display: '', left: null, top: false, fontFamily: true};
      ReactDOM.render(<div style={reset} />, container);
      expect(stubStyle.display).toEqual('');
      expect(stubStyle.left).toEqual('');
      expect(stubStyle.top).toEqual('');
      expect(stubStyle.fontFamily).toEqual('');
    });
    it('should not update styles when mutating a proxy style object', () => {
      const styleStore = {
        display: 'none',
        fontFamily: 'Arial',
        lineHeight: 1.2,
      };
      
      
      const styles = {
        get display() {