用图片背景代替文本时，为什么不推荐用textindent:9999em？

@mixin hide-text($direction: $hide-text-direction) {

  @if $direction == left {

    $approximate-em-value: 12px / 1em;

    $wider-than-any-screen: -9999em;

    text-indent: $wider-than-any-screen * $approximate-em-value;

    overflow: hidden;

    text-align: left;

  }

  @else {

    // slightly wider than the box prevents issues with inline-block elements

    text-indent: 110%;

    white-space: nowrap;

    overflow: hidden;

  }

  @include for-legacy-browsers((ie: "7"), $critical-usage-threshold) {

    //Text transform capitalize fixes text-replacement issue when used in a element on ie7

    text-transform: capitalize;

  }

}