css - Padding-bottom/top in flexbox layout

Question

I have a flexbox layout containing two items. One of them uses padding-bottom :

#flexBox {
  border: 1px solid red;
  width: 50%;
  margin: 0 auto;
  padding: 1em;
  display: flex;
  flex-direction: column;
}
#text {
  border: 1px solid green;
  padding: .5em;
}
#padding {
  margin: 1em 0;
  border: 1px solid blue;
  padding-bottom: 56.25%; /* intrinsic aspect ratio */
  height: 0;
}

<div id='flexBox'>
  <div id='padding'></div>
  <div id='text'>Some text</div>
</div>

Answer 1

Update september 2020

FireFox and edge have implemented the behaviour from the specs and margin + padding for flex elements are both calculated occording to the width of the containing block.
Just like block elements.

Update Febuary 2018

Firefox and edge have agreed to change their behaviour on top, bottom margin and padding for flex (and grid) items :

[...] e.g. left/right/top/bottom percentages all resolve against their containing block’s width in horizontal writing modes. [source]

This is not yet implemented (tested on FF 58.0.2).

Update april 2016

(still valid in may 2017)

The specs have been updated to:

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)

source: CSS Flexible Box Layout Module Level 1

This means that chrome IE FF and Edge (even if they don't have the same behaviour) follow the specs recomendation.

Specs also say:

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

---

Work around :

You can wrap the first child of the flex container in an other element and put the padding-bottom on the second child :

#flexBox {
  border: 1px solid red;
  width: 50%;
  margin: 0 auto;
  padding: 1em;
  display: flex;
  flex-direction: column;
}
#text {
  border: 1px solid green;
  padding: .5em;
}
#padding {
  margin: 1em 0;
  border: 1px solid blue;
}
#padding > div {
  padding-bottom: 56.25%; /* intrinsic aspect ratio */
}

<div id='flexBox'>
  <div id='padding'><div></div></div>
  <div id='text'>Some text</div>
</div>