<div class="block">
  <div class="line"></div>
</div>
.block  {
  width: 500px;
  height: 500px;
  background-color: black;
  position: relative;
  overflow: hidden;
}

.line {
  width: 100%;
  height: 50px;
  background-color: yellow;
  transform: rotate(135deg);
  position: absolute;
  bottom: 50px;
  right: -100px;
}

External CSS

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External JavaScript

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