body {
  margin: 0;
  padding: 0;
}

main {
  width: 100%;
  max-width: 700px;
  margin: auto;
  padding: 0 20px;
  position: relative;
  box-sizing: border-box;
}

main div.sticky {
  width: 100%;
  padding: 10px;
  position: sticky;
  top: 10px;
  background-color: steelblue;
  box-sizing: border-box;
  border: 1px solid black;
  color: white;
}