# 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I would like to close those PRs as outdated, and reboot the discussion about this.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 NORMALIZE_NEWLINES_REGEX = /\r\n?/g;
  const NORMALIZE_NULL_AND_REPLACEMENT_REGEX = /\u0000|\uFFFD/g;
  normalizeMarkupForTextOrAttribute = function(markup: mixed): string {
    const markupString =
      typeof markup === 'string' ? markup : '' + (markup: any);
    return markupString
      .replace(NORMALIZE_NEWLINES_REGEX, '\n')
      .replace(NORMALIZE_NULL_AND_REPLACEMENT_REGEX, '');
  };
  warnForTextDifference = function(
    serverText: string,
    clientText: string | number,
  ) {
    if (didWarnInvalidHydration) {
      return;
    }
    const normalizedClientText = normalizeMarkupForTextOrAttribute(clientText);
    const normalizedServerText = normalizeMarkupForTextOrAttribute(serverText);
    if (normalizedServerText === normalizedClientText) {
      return;
    }
    didWarnInvalidHydration = true;
    warningWithoutStack(
      false,
      'Text content did not match. Server: "%s" Client: "%s"',
      normalizedServerText,
      normalizedClientText,