Set same height and responsiveness for div only css

Question

I have two divs and they should be the same size. I tried to put a height it works in desktop size but when I use different platform it become a mess

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.custombox {
  text-align: center;
  padding: 10px;
  position: absolute;
  border: 1px solid;
}

.customparentbox {
  background: red
}

<div class="customparentbox">
  <div class="custombox">
    <p>Sample text</p>
    <button>test</button>
  </div>
</div>

Answer 1

I've made some changes in your css

From

.custombox{
  text-align: center;
  padding: 10px;
  position: absolute;
  border: 1px solid;}
  .customparentbox{background:red}

To

.custombox {
  text-align: center;
  padding: 10px;
  position: relative;
  top:10px;
  left:10px;
  border: 1px solid;
}

.customparentbox {
  width:100px;
  background: red;
  border:1px solid green;
}

Here is a jsfiddle I made.