<div class="container">
  <div class="item">
    Item 1
  </div>
  <div class="item">
    Item 2
  </div>
  <div class="item">
    Item 3
  </div>  
</div>
.container {
  display: flex;
  color: red;
}

.item {
  flex: 1;
}

@supports (display:grid) {
  .container {
    color: green;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
  }
}

/* ETC (Formatting only): */

body {
  margin: 1.5em;
  font-size: 130%;
  font-family: sans-serif;
}

.container {
  border: 5px solid black;
  height: 10em;
}

.item {  
  padding: 1em; 
  text-align: center;
  background: lightgrey;
  border: 1px solid dimgrey;  
}

External CSS

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External JavaScript

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