CSS generated content can solve this for you:
div {
position: relative;
}
/* Main div for border to extend to 50% from bottom left corner */
div:after {
content: "";
background: black;
position: absolute;
bottom: 0;
left: 0;
height: 50%;
width: 1px;
}
<div>Lorem Ipsum</div>
(note – the content: "";
declaration is necessary in order for the pseudo-element to render)