add clas do element code example

Example 1: js add class

var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');

Example 2: add class to html tag javascript

var root = document.getElementsByTagName( 'html' )[0]; // '0' to assign the first (and only `HTML` tag)

root.className += ' responsive';