Try it Live

​x
 
<!DOCTYPE html>
<html>
  <head>
    <style>
     .border-bot-grad{
       border-bottom: 10px solid;
       border-image: linear-gradient(to right, hotpink, lawngreen);
       border-image-slice: 1;
      }
    </style>
  </head>
  <body class="border-bot-grad">
    <h3>Example border-bottom gradient</h3>
  </body>
</html>
​