the canvas can resolve its percentage against that,
  and thus the intrinsic flex basis results in a flex base size of 100px.">

<style>
.container {
  display: inline-flex;
  vertical-align: top;
  width: 50px;
  height: 50px;
  background: red;
  margin: 5px;
}
.container.column {
  flex-direction: column;
}
.item {
  min-width: 0;
  min-height: 0;
  background: green;
}
.container.column .stretch-size {
  width: -moz-available;
  width: -webkit-fill-available;
  width: stretch;
}
.container.row .stretch-