CSS Absolute and Relative Units

Both CSS Absolute Units and Relative Units fall under the category Distance units.
CSS Relative Units define a length of an element in reference to another element.
For example, vh relative unit is relative to viewport height.
Following are the CSS relative units −
Let us see an example for CSS relative units −
 Live Demo
<!DOCTYPE html>
<html>
<head>
<title>CSS Relative Units</title>
<style>
html{
   font-size: 14px;
   line-height: normal;
}
form {
   width:70%;
   margin: 0 auto;
   text-align: center;
}
* {
   padding: 2px;
   margin:5px;
}
input[type="button"] {
   border-radius: 10px;
}
#textContain {
   font-size: 20px;
   line-height: 2;
}
</style>
</head>
<body>
<form>
<fieldset>
<legend>CSS-Relative-Units</legend>
<input type="text" id="textSelect" placeholder="Type Text Here...">
<input type="number" id="numSelect" value="1" min="1">
<div id="radioBut">
<input class="radio" type="radio" name="heading" value="em" checked><label>em</label>
<input class="radio" type="radio" name="heading" value="rem"><label>rem</label>
<input class="radio" type="radio" name="heading" value="vw"><label>vw</label>
<input class="radio" type="radio" name="heading" value="vh"><label>lh</label>
<input class="radio" type="radio" name="heading" value="ex"><label>ex</label>
</div>
<div id="textContain">Text Preview: <span id="textPreview">Output will show up here</span></div>
<input type="button" onclick="changeText()" value="Preview">
</fieldset>
</form>
<script>
var textSelect = document.getElementById("textSelect");
var numSelect = document.getElementById("numSelect");
var textPreview = document.getElementById("textPreview");
function changeText() {
   if(textSelect.value === '')
      textPreview.textContent = 'Type some text first';
   else{
      for(var i=0; i<5; i++){
         var radInp = document.getElementsByClassName('radio')[i];
         if(radInp.checked === true){
            textPreview.textContent = textSelect.value;
            textPreview.style.fontSize = numSelect.value+radInp.value
         }
      }
   }
}
</script>
</body>
</html>
Following is the output for above code −
Before clicking any button −
After clicking ‘Preview’ button with ‘em’ option selected and empty text field −
After clicking ‘Preview’ button with ‘rem’ option selected and non-empty text field −
After clicking ‘Preview’ button with ‘em’ option selected and non-empty text field −
Following are the CSS absolute units −
Let us see an example for CSS absolute units −
 Live Demo
<!DOCTYPE html>
<html>
<head>
<title>CSS Absolute Units</title>
<style>
form {
   width:70%;
   margin: 0 auto;
   text-align: center;
}
* {
   padding: 2px;
   margin:5px;
}
input[type="button"] {
   border-radius: 10px;
}
</style>
</head>
<body>
<form>
<fieldset>
<legend>CSS-Absolute-Units</legend>
<input type="text" id="textSelect" placeholder="Type Text Here...">
<input type="number" id="numSelect" value="10" min="1">
<div id="radioBut">
<input class="radio" type="radio" name="heading" value="pc"><label>pc</label>
<input class="radio" type="radio" name="heading" value="pt"><label>pt</label>
<input class="radio" type="radio" name="heading" value="px" checked><label>px</label>
<input class="radio" type="radio" name="heading" value="in"><label>in</label>
<input class="radio" type="radio" name="heading" value="cm"><label>cm</label>
</div>
<div id="textContain">Text Preview: <span id="textPreview">Output will show up here</span></div>
<input type="button" onclick="changeText()" value="Preview">
</fieldset>
</form>
<script>
var textSelect = document.getElementById("textSelect");
var numSelect = document.getElementById("numSelect");
var textPreview = document.getElementById("textPreview");
function changeText() {
   if(textSelect.value === '')
      textPreview.textContent = 'Type some text first';
   else{
      for(var i=0; i<5; i++){
         var radInp = document.getElementsByClassName('radio')[i];
         if(radInp.checked === true){
            textPreview.textContent = textSelect.value;
            textPreview.style.fontSize = numSelect.value+radInp.value
         }
      }
   }
}
</script>
</body>
</html>
Following is the output for above code −
Before clicking any button −
After clicking ‘Preview’ button with ‘px’ option selected and non-empty text field −
After clicking ‘Preview’ button with ‘pt’ option selected and non-empty text field −
After clicking ‘Preview’ button with ‘pc’ option selected and non-empty text field −