Run Example

Edit This Code:
<!DOCTYPE html>
<html>
 <head>
  <title> Page Title </title>
  <style type="text/css">
   p.px{
   	font-size: 20px;
   }
   p.percent{
   	font-size: 100%;
   }
   p.em{
   	font-size: 2.5em;
   }
  </style>
 </head>

 <body>
   <p class="px">This is a paragraph of 20px font size</p> 
   <p class="percent">This is a paragraph of 100% font size</p> 
   <p class="px">This is a paragraph of 2.5em font size</p> 
 </body>
</html>
Result: