<!DOCTYPE html>
<meta charset="utf-8">
<style>
.flex-container {
  display: flex;
  height: 100vh;
}

.column {
  flex-direction: column;
}

.flex-child {
  height: 100%;
  width: 100%
}
</style>
<div class="flex-container">
  <div class="flex-child"></div>
  <div id="flex-item-with-unauthored-min-width">
      <h1>AAA</h1>
  </div>
</div>
<div class="flex-container column">
  <div class="flex-child"></div>
  <div id="flex-item-with-unauthored-min-height">
      <h1>BBB</h1>
  </div>
</div>
