<div class="block"></div>

*{
margin: 0;
padding: 0;
box-sizing: border-box;
}

.block{
  --blockOffset: 33vw;
  position: relative;
  left: var(--blockOffset);
  width: 300px;
  height: 50px;
  background-color: red;
}
.block::after{
    content: '';
    display: block;
    position: absolute;
    left: 0;
    bottom: 0;
    height: 5px;
    width: calc(100vw - var(--blockOffset));
    background-color: green;
}