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

 ____________
|  ___________                 
| |         | |
| |         | |  
| |         | |
|_|_________  |
  |___________|

.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.

Answer 2

Here's my working jsFiddle.

You have to add height and width to your parent div element and also to the child div. I have added box-sizing in the style so that it will render the actual width and height of the element.

Add position:relative to the parent element.

*{ box-sizing: border-box; margin: 0; padding: 0; }

.customparentbox{
background: red;
position: relative;
height: 80px;
width: 100px;
}
.custombox{
text-align: center;
padding-top: 10px;
position: absolute;
border: 1px solid;
left: 10px;
top: 10px;
height: 80px;
width: 100px;
background: #fff; }

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