add class to html tag javascript

// getting the target element
var myElement = document.querySelector('#myElement');

// example for addding some-class to the element
myElement.classList.add('some-class');

// multiple classes can be added like this
myElement.classList.add('one-class', 'one-more-class');

// example for removing any class from the element
myElement.classList.remove('any-class');

var element = document.getElementById("myDIV");
element.classList.add("mystyle"); 

function myFunction() {
  var element = document.getElementById("myDIV");
  element.classList.add("mystyle");
}

var someElement= document.getElementById("myElement");
    someElement.className += " newclass";//add "newclass" to element (space in front is important)

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

root.className += ' responsive';