How to create a button — add to home screen/bookmark (mobile/desktop)?

function bookmark(a) {
title=document.title;
url=document.location;
 try { 
 window.sidebar.addPanel(title, url, "");
 } catch (e) {
 if (window.opera && window.print || typeof(window.sidebar)=="object") { // Opera || Mozilla
a.rel="sidebar";
a.title=title;
a.url=url;
a.href=url;
 return true;
 } else if(document.all) { // Internet Explorer
 window.external.AddFavorite(url, title);
 return true;
 } else {
 alert('Press Ctrl+D (Cmd+D on MacOS) to add the page to favorites');
}
}
}