[css] How to make borders collapse (on a div)?

You could also use negative margins:

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_x000D_
.column {_x000D_
  float: left;_x000D_
  overflow: hidden;_x000D_
  width: 120px;_x000D_
}_x000D_
.cell {_x000D_
  border: 1px solid red;_x000D_
  width: 120px;_x000D_
  height: 20px;_x000D_
  box-sizing: border-box;_x000D_
}_x000D_
.cell:not(:first-child) {_x000D_
  margin-top: -1px;_x000D_
}_x000D_
.column:not(:first-child) > .cell {_x000D_
  margin-left: -1px;_x000D_
}
_x000D_
<div class="container">_x000D_
  <div class="column">_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
  </div>_x000D_
  <div class="column">_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
  </div>_x000D_
  <div class="column">_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
  </div>_x000D_
  <div class="column">_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
  </div>_x000D_
  <div class="column">_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
    <div class="cell"></div>_x000D_
  </div>_x000D_
</div>
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