<html><head><script src="https://libs.baidu.com/jquery/2.1.4/jquery.min.js"></script><script>$(document).ready(function(){ $("button").click(function(){ $("p:first").addClass("intro note"); });});</script><style>.intro { font-size: 30px; color: blue;}.note { background-color: yellow;}</style></head><body><h1>This is a heading</h1><p>This is a paragraph.</p><p>This is another paragraph.</p><button>Add two class names to the first p element</button></body></html>