Change the width of the bottom border of a <div> element to thin:
document.getElementById("myDiv").style.borderBottomWidth = "thin";
<!DOCTYPE html> <html> <head> <style> #myDiv {/* www .j a v a2 s . c om*/ border-style: solid; } </style> </head> <body> <div id="myDiv">This is a div element.</div> <br> <button type="button" onclick="myFunction()">Change width of the bottom border</button> <script> function myFunction() { document.getElementById("myDiv").style.borderBottomWidth = "thin"; } </script> </body> </html>