CSS Hypot Function Basic

<!DOCTYPE html>
<title>My Example</title>

.diagonal-line {
    position: absolute;
    top: 0;
    left: 0;
    height: 4px;
    background-color: #dc3545;
    transform-origin: top left;
    
    /* 
      1. Use hypot() to calculate the exact diagonal length.
      2. Use atan2() to calculate the exact angle to point to the bottom-right corner.
    */
    width: hypot(var(--box-width), var(--box-height));
    transform: rotate(atan2(var(--box-height), var(--box-width)));
  }
  
  .label {
    position: absolute;
    bottom: 10px;
    left: 10px;
    font-family: sans-serif;
    font-size: 0.9em;
    color: #6c757d;
  }
</style>

<div class="hypot-container">
  <div class="diagonal-line"></div>
  <div class="label">Width: 300px | Height: 150px</div>
</div>