@charset "utf-8";

*, *::before, *::after {
  box-sizing: border-box;
}
body {
  margin: 0;
  padding: 0;
}

.contents-before {
  width: calc(100% - 100px);
  margin-inline: 50px;
  background-color: tomato;

  margin-block: 30px;
}
.contents-after {
  width: -webkit-fill-available;
  width: -moz-available;
  width: stretch;
  margin-inline: 50px;
  background-color: aqua;

  margin-block: 30px;
}