In order to achieve this, you should consider altering your markup in order to make your html more efficient. This can be achieved using a pseudo element. I'll address each point individually, and put it all together at the end of my answer.
First of all,
Use pseudo elements to avoid extra elements
You could use a pseudo element to remove the extra .triangle
div. This not only reduces your div numbers, but also helps with positioning as you can use the top:
left:
right:
and bottom:
css properties in order to position according to your main element. This can be seen below:
.oneAndOnlyDiv {
height: 100px;
width: 200px;
border: 3px solid gray;
background: lightgray;
position: relative;
}
.oneAndOnlyDiv:before {
content: "";
position: absolute;
top: 100%;
left: 20px;
width: 0;
border-top: 20px solid black;
border-left: 20px solid transparent;
border-right: 20px solid transparent;
}
<div class="oneAndOnlyDiv">Main div</div>
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