Треугольник отдельно сверстан дивом + :before, можно сделать, используя в :before свойство transform
.comment {
position: relative; }
.comment .block {
position: relative;
border: 1px cadetblue solid;
z-index: 1;
height: 100px;
width: 400px;
border-bottom-left-radius: 10px;
border-bottom-right-radius: 10px;
border-top-right-radius: 10px; }
.comment .block__triangle {
top: -12px;
display: block;
height: 12px;
width: 20px;
z-index: 2;
border-bottom: white 1px solid;
border-left: solid 1px cadetblue;
border-top: 0;
position: absolute; }
.comment .block__triangle:after {
top: -15px;
content: "";
display: block;
border-bottom-left-radius: 150px;
border-left: solid 1px cadetblue;
border-bottom: 1px solid cadetblue;
border-top: transparent;
height: 12px;
width: 20px; }