web/style.css

body {
  margin: 40px;
}

h2 {
  font: 24px normal Georgia, serif;
}

pre, textarea {
  font: 14px monospace;
}

pre, textarea {
  margin: 0;
  padding: 0;
}

.code-area {
  position: relative;
  background: hsl(220, 10%, 95%);
  z-index: 1;
  width: 800px;
  overflow: hidden;
}

.column-marker {
  position: absolute;
  left: 0;
  right: 0;
  z-index: 1;

  color: hsl(220, 10%, 80%);
}

textarea {
  position: relative;
  z-index: 2;

  border: none;
  outline: none;
  background: none;
  color: hsl(220, 10%, 40%);
}