Tutorial addClass Function
If you are going library-free, you might need to roll your own function like this. function addClass(id,new_class){ var i,n=0; ...

https://iskablogs.blogspot.com/2014/03/tutorial-addclass-function.html
If you are going library-free, you might need to roll your own function like this.
function addClass(id,new_class){
var i,n=0;
new_class=new_class.split(",");
for(i=0;i if((" "+document.getElementById(id).className+" ").indexOf(" "+new_class[i]+" ")==-1){
document.getElementById(id).className+=" "+new_class
function addClass(id,new_class){
var i,n=0;
new_class=new_class.split(",");
for(i=0;i
document.getElementById(id).className+=" "+new_class