<!DOCTYPE html>
<html>
  <style>
  fieldset.outer {
    column-count: 1;
    transform: translateX(3px);
    position: relative;
    width: 100px;
    height: 50px;
    border: 1px solid blue;
    margin: 0;
  }

  div.fixed {
    position: fixed;
    top: 0;
    left: 0;
    background: green;
  }
  </style>

  <!-- ABC should be inside the fieldset since 'transform' creates a stacking context. -->
  <fieldset class="outer">
    <legend>Legend</legend>
    <div class="fixed">ABC</div>
  </fieldset>
</html>