<div>
<div class="square1"/>
<div class="square2"/>
</div>
.square1 {
  width: 50px;
  height: 50px;
  background-color: red;
  border: 1px solid black;
  
  animation: increaseBorderSize 2s both infinite;
}

@keyframes increaseBorderSize {
  0% {
    border: 1px solid black;
  }
  100% {
    border: 10px solid black;
  }
}

.square2 {
  width: 50px;
  height: 50px;
  background-color: orange;
  margin-left: 100px;
}

External CSS

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External JavaScript

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