<html><head><script src="https://libs.baidu.com/jquery/2.1.4/jquery.min.js"></script><script>$(document).ready(function(){ $("button").click(function(){ $("#div1").addClass("important blue"); });});</script><style>.important { font-weight: bold; font-size: xx-large;}.blue { color: blue;}</style></head><body><div id="div1">This is some text.</div><div id="div2">This is some text.</div><br><button>Add classes to first div element</button></body></html>