SOURCE

<div class="wrap">
  你好，我是一条线
</div>

div{
	box-sizing:border-box;
}
.wrap{
	padding:10px;
background-color:#fff;
	margin-left: 5xp;
	position:relative;
	border-radius:5px;
}

.wrap::before{
	content:'';
	display:block;
	background:#3eaf7c;
	position:absolute;
	z-index:-1;
	left:-5px;
	top:0;
	width:20px;
	height:100%;
	border-radius:5px 0 0 5px;
}