Why doesn't percentage padding / margin work on flex items in Firefox and Edge?

Answer 1

2018 Update

The flexbox specification has been updated.

4.2. Flex Item Margins and Paddings

Percentage margins and paddings on flex items, like those on block boxes, are resolved against the inline size of their containing block, e.g. left/right/top/bottom percentages all resolve against their containing block’s width in horizontal writing modes.

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Original Answer - applies to FF and Edge versions released before 2018

From the flexbox specification:

Authors should avoid using percentages in paddings or margins on flex items entirely, as they will get different behavior in different browsers.

Here's some more:

4.2. Flex Item Margins and Paddings

Percentage margins and paddings on flex items can be resolved against either:

• their own axis (left/right percentages resolve against width, top/bottom resolve against height), or,
• the inline axis (left/right/top/bottom percentages all resolve against width)

A User Agent must choose one of these two behaviors.

Note: This variance sucks, but it accurately captures the current state of the world (no consensus among implementations, and no consensus within the CSSWG). It is the CSSWG’s intention that browsers will converge on one of the behaviors, at which time the spec will be amended.

Answer 2

In addition to Michael_B's answer, here is a possible workaround.

When using percent we often relate that to the viewport width, so with that in mind, viewport units vw/vh can be an option, since it works similar (responsive).

Stack snippet

.outer {
  display: flex;
  width: 100%;
  background: blue;
}
.inner {
  width: 50%;
  background: yellow;
  padding-bottom: 50vw;
}

<div class="outer">
  <div class="inner">
    <a>hehe</a>
  </div>
</div>

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Updated based on a comment

If a square is a must, and viewport units or script can't be used, here is another trick using a dummy image.

Note, as image also a SVG or a Base64 could be used as a datauri to save an extra round trip to the server

.outer {
  display: flex;
  width: 100%;
  background: blue;
}
.inner {
  width: 50%;
  background: yellow;
}
.inner img {
  display: block;
  width: 100%;
  visibility: hidden;
}

<div class="outer">
  <div class="inner">
    <img src="http://placehold.it/10" alt="">
  </div>
</div>