<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title></title> <style type="text/css"> .bgred{ background: red; } .bgblue{ background: skyblue; } </style> </head> <body> <h1>helloworld</h1> <script type="text/javascript"> var h1 = document.querySelector('h1') // h1.style.borderBottom = '11px solid #eee' h1.className = 'bgred' </script> </body> </html>