<!DOCTYPE html>
<style>
.parent div {
  background: #aaa;
  width: 100%;
  height: 100px;
}
</style>

There should be two grey ~squares with no red showing.

<div class="parent" style="width:100.2px">
  <div></div>
</div>

<div class="parent" style="width:100.8px">
  <div></div>
</div>