两个三角形重叠，a作背景，b作边框，定位相差1px
<style type="text/css">
.message-box {
position: relative;
width: 240px;
height: 60px;
line-height: 60px;
background: #E9FBE4;
box-shadow: 1px 2px 3px #E9FBE4;
border: 1px solid #C9E9C0;
border-radius: 4px;
text-align: center;
color: #0C7823;
}
.triangle-border {
position: absolute;
left: 30px;
overflow: hidden;
width: 0;
height: 0;
border-width: 10px;
border-style: solid dashed dashed dashed;
}
.tb-border {
bottom: -20px;
border-color: #C9E9C0 transparent transparent transparent;
}
.tb-background {
bottom: -19px;
border-color: #E9FBE4 transparent transparent transparent;
}
</style>
<body>
<div class="message-box">
<div class="triangle-border tb-border"></div>
<div class="triangle-border tb-background"></div>
</div>
</body>