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I would like to close those PRs as outdated, and reboot the discussion about this.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

'whatever="false" or whatever={value.toString()}.\n\n' +
          'If you used to conditionally omit it with whatever={condition && value}, ' +
          'pass whatever={condition ? value : undefined} instead.',
      );
      expect(el.querySelector('font-face').hasAttribute('whatever')).toBe(
        false,
      );
    });
  });
  
  
  describe('Custom elements', () => {
    it('does not strip unknown boolean attributes', () => {
      const container = document.createElement('div');
      ReactDOM.render(<some-custom-element foo={true} />, container);
      const node = container.firstChild;
      expect(node.getAttribute('foo')).toBe('true');
      ReactDOM.render(<some-custom-element foo={false} />, container);
      expect(node.getAttribute('foo')).toBe('false');
      ReactDOM.render(<some-custom-element />, container);
      expect(node.hasAttribute('foo')).toBe(false);
      ReactDOM.render(<some-custom-element foo={true} />, container);
      expect(node.hasAttribute('foo')).toBe(true);
    });
    it('does not strip the on* attributes', () => {
      const container = document.createElement('div');
      ReactDOM.render(<some-custom-element onx="bar" />, container);
      const node = container.firstChild;
      expect(node.getAttribute('onx')).toBe('bar');
      ReactDOM.render(<some-custom-element onx="buzz" />, container);