<section><div>Block element</div></section>
<section><span>Inline element</span></section>
<section class="box"><div>Content box</div></section>
<section class="box borderbox"><div>Border box</div></section>

index.html

html {
  text-align: center;
}
body {
  margin: 0;
}
section {
  padding: 1rem;
  margin: 1rem;
  background-color: springgreen;
  border: 1rem solid blue;
}
div, span {
  background-color: yellow;
}
.box {
  height: 100px;
  width: 100px;
}
.borderbox {
  box-sizing: border-box;
}

index.css