As per your request for a taller border (red one)....
...utilize the pseudo class :after with position:absolute
.
...set your div width and height equal to the border-width, which is 100px, and apply box-sizing:border-box
.
...the height value is determined by the square root of 1002 + 1002(since 100 is the border-width) plus extra width as needed (e.g., 6 in this scenario).
...set the left
value to half of the border-width
...the top
value will be (border-width+height)/2
.
Snippet for demonstration purposes
.left-corner {
width: 100px;
height: 100px;
border-top: 100px solid powderblue;
border-left: 100px solid transparent;
position: relative;
box-sizing: border-box;
}
.left-corner:after {
content: "";
position: absolute;
top: -123.4213px;
left: -52px;
width: 6px;
background: red;
height: 147.4213px;
transform: rotate(-45deg);
border-radius: 4px;
}
<div class="left-corner"></div>