Actually we can create the “triangle part” in border itself by using the :before or :before pseudo-element. Check the below codes for reference.
Method 1:
<!DOCTYPE html>
<html>
<head>
<style>
body {
background-color: #F3F5F6;
width: 500px;
margin: 0 auto;
padding: 10%;
}
.message {
display: block;
position: relative;
background: #FFFFFF;
padding: 15px;
border: 1px solid #DDDDDD;
margin-top: 20px;
}
.message:after {
content: '';
display: block;
position: absolute;
left: 20px;
bottom: 100%;
width: 0;
height: 0;
border-bottom: 10px solid #FFFFFF;
border-top: 10px solid transparent;
border-left: 10px solid transparent;
border-right: 10px solid transparent;
}
</style>
</head>
<body>
<h1>Create triangle shape for div Method 1</h1>
<div class="message">
Testing
</div>
</body>
</html> Method 2:
<!DOCTYPE html>
<html>
<head>
<style>
body {
background-color: #F3F5F6;
width: 500px;
margin: 0 auto;
padding: 10%;
}
.message {
display: block;
position: relative;
background: #FFFFFF;
padding: 15px;
border: 1px solid #DDDDDD;
margin-top: 20px;
}
.message:before, .message:after {
content: '';
display: block;
position: absolute;
bottom: 100%;
width: 0;
height: 0;
}
.message:before {
left: 19px;
border: 11px solid transparent;
border-bottom-color: #ddd;
}
.message:after {
left: 20px;
border: 10px solid transparent;
border-bottom-color: #fff;
}
</style>
</head>
<body>
<h1>Create triangle shape for div Method 2</h1>
<div class="message">
Testing
</div>
</body>
</html> 
