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chromium/third_party/blink/web_tests/external/wpt/css/css-flexbox/percentage-padding-001.html 

<!DOCTYPE html>
<script src='/resources/testharness.js'></script>
<script src='/resources/testharnessreport.js'></script>
<script src='/resources/check-layout-th.js'></script>
<link rel="author" title="David Grogan" href="[email protected]">
<link rel="help" href="https://drafts.csswg.org/css-box-3/#padding-physical" title="Percentages line">
<meta name="assert" content="item's padding is resolved against parent's size, not item's" />

<style>
x-flexbox {
  display: flex;
  height: 50px;
  width: 500px;
}

x-item {
  padding-left: 10%;
  background: blue;
  position: relative; /* Just so offsetLeft returns distance between x-item and div */
}

div {
  height: 50px;
  width: 50px;
  background: orange;
}
</style>

If the item's % padding is calculated correctly, there is a blue square on the left and an orange square on the right, both 50x50.

<x-flexbox>
  <x-item>
    <div data-offset-x=50></div>
  </x-item>
</x-flexbox>

<script>
checkLayout('x-flexbox');
</script>