Change the style of the bottom border of a <div> element to "dotted":
document.getElementById("myDiv").style.borderBottomStyle = "dotted";
<!DOCTYPE html> <html> <head> <style> #myDiv {/*ww w. ja v a 2 s. c o m*/ border: thick solid #FF0000; } </style> </head> <body> <div id="myDiv">This is a div.</div> <br> <button type="button" onclick="myFunction()">Change style of the bottom border</button> <script> function myFunction() { document.getElementById("myDiv").style.borderBottomStyle = "dotted"; } </script> </body> </html>