JSFiddle - React, Tailwind, and code Playground

HTML

<main class="main">
  <div class="block">sdfsdf</div>
  <div class="block">sdfsdf</div>
  <div class="block">sdfsdf</div>
</main>
<footer class="footer">
  <div class="footer__angle"></div>
</footer>

CSS

body {
  display: flex;
  height: 100%;
  background-color: gray;
  padding-bottom: 100px;
}

.main {
  display: flex;
  flex-direction: column;
}

.block {
  display: flex;
  height: 600px;
  background-color: green;
  width: 100%;
  margin-bottom: 10px;
}

.footer {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  height: 100px;
  background-color: #000;
}

.footer__angle {
  position: absolute;
  top: -15px;
  left: 0;
  height: 15px;
  width: 100%;
  overflow: hidden;
  pointer-events: none;
}

.footer__angle:before {
  content: '';
  display: block;
  height: 100%;
  width: 100%;
  border-radius: 0 0 15px 15px;
  box-shadow: 0 0 0 30px black;
}