<div class="flex">
  <div class="thin">border-width: thin;</div>
  <div class="medium">border-width: medium;</div>
  <div class="thick">border-width: thick;</div>
</div>
.flex {
  display: flex;
  flex-wrap: wrap;
}

.flex div {
  min-width: 200px;
  height: 100px;
  text-align: center;
  margin: 10px; 0;
  line-height: 100px;
  border-style: solid;
}

.thin {
  border-width: thin;
}
.medium {
  border-width: medium;
}

.thick {
  border-width: thick;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.