<div class="solid"></div>
<div class="outline"></div>
<div class="pseudo elements"></div>
<div class="linear-gradient"></div>
<div class="box-shadow"></div>
<div class="border-image"></div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  padding: 10px;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(30vw, 1fr));
  grid-gap: 10px;
}

div {
  height: 150px;
  color: #000;
  position: relative;
  
  &::before {
    content: attr(class);
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    font: 600 1.5em/ 1.375 segoe script, comic sans ms, cursive;
	  text-shadow: 1px 1px #000;
    white-space: nowrap;
  }
  
  &[class="solid"] {
    border: 1em solid currentColor;
  }
  
  &[class="outline"] {
    margin: 1em;
    height: calc(150px - 2em);
    outline: 1em solid;
  }
  
  &[class="pseudo elements"] {
    background: linear-gradient(to right, #fff, #fff);
    position: relative;
    padding: 1em;
    background-clip: content-box;
    
    &::after {
      content: '';
      position: absolute;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      background: #000;
      z-index: -1;
    }
  }
  
  &[class="linear-gradient"] {
    padding: 1em;
    background: linear-gradient(to right, #fff, #fff), #000;
    background-clip: content-box, padding-box;
  }
  &[class="box-shadow"] {
    width: calc(100% - 2em);
    height: calc(150px - 2em);
    margin: 1em auto;
    box-shadow: 0 0 0 1em #000;
  }
  &[class="border-image"] {
    border: 1em solid transparent;
    border-image: linear-gradient(to right, #000, #000) 10% round;
  }
}