Totally Triangle

<div></div>
<style>
  body {
    background: #0b2429;
    margin: 0;
  }
  div {
    width: 140px;
    height: 140px;
    background: #f3ac3c;
    clip-path: polygon(0 100%, 100% 0%, 0% 0%);
  }
</style>