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<!DOCTYPE html> <html> <head> <script src="/lib/jquery-1.12.2.min.js"></script> <script> $(document).ready(function(){ $("button").click(function(){ $("p:first").addClass("intro"); }); }); </script> <style> .intro { font-size: 150%; color: red; } </style> </head> <body> <h1>This is a heading</h1> <p>This is a paragraph.</p> <p>This is another paragraph.</p> <button>Add a class name to the first p element</button> </body> </html>