Например вот так
HTML:<div class="block">
some text
<span class="triangle"></span>
</div>
CSS:.block {
width: 400px;
padding: 10px;
background: #fff;
min-height: 100px;
position: relative;
border: 1px dashed #aaa;
margin: 10px 10px 10px 40px;
}
.block span.triangle {
width: 0;
height: 0;
display: block;
position: absolute;
top: 30px; left: -30px;
border-bottom: 30px solid #fff;
border-left: 30px solid transparent;
}
.block span.triangle:before {
content: '';
width: 40px;
display: block;
position: absolute;
top: 15px; left: -35px;
border-top: 1px dashed #aaa;
-webkit-transform: rotate(-45deg);
transform: rotate(-45deg);
}
.block span.triangle:after {
content: '';
width: 30px;
display: block;
position: absolute;
top: 30px; left: -30px;
border-top: 1px dashed #aaa;
}
jsfiddle.net