Change Text Size HTML/JavaScript

HTML code to add buttons

Add id="body" to the body tag.

  <body id="body" >

Add code for the buttons:

  <button type="button" onclick="bigger()">Bigger</button>
  <button type="button" onclick="smaller()">Smaller</button>

JavaScript Code to change font size:

    <script>
        function bigger(){
            document.getElementById('body').style.fontSize='x-large';
        }
        function smaller(){
            document.getElementById('body').style.fontSize='medium';
        }
    </script>