<div class="grid__container">
  <div class="grid__item">A</div>
  <div class="grid__item">B</div>
  <div class="grid__item">C</div>
  <div class="grid__item grid__container--nested">
    <div class="nested__item grid__item">1</div>
    <div class="nested__item grid__item">2</div>
    <div class="nested__item grid__item">3</div>
    <div class="nested__item grid__item">4</div>

  </div>
  <div class="grid__item">E</div>
  <div class="grid__item">F</div>
  <div class="grid__item">F</div>
  <div class="grid__item">H</div>
</div>

<div class="grid__container">
  <div class="grid__item">A</div>
  <div class="grid__item">B</div>
  <div class="grid__item">C</div>
  <div class="grid__item grid__container--subgrid">
    <div class="subgrid__item grid__item">1</div>
    <div class="subgrid__item grid__item">2</div>
    <div class="subgrid__item grid__item">3</div>
    <div class="subgrid__item grid__item">4</div>
  </div>
  <div class="grid__item">E</div>
  <div class="grid__item">F</div>
  <div class="grid__item">F</div>
  <div class="grid__item">H</div>
</div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

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

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  padding: 4rem;
}

.warning {
  position: fixed;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: 2;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 0.5rem;
  background-color: #fed7d7;
  border-top: 1px solid #f56565;
  padding: 10px;
  color: #f56565;
}

@supports (grid-template-rows: subgrid) {
  .warning {
    display: none;
  }
}

.grid__container {
  height: calc(100vh - 8rem);
  max-width: 1024px;
  margin: 0 auto;
  outline: 3px solid #111;
  background: #333;
  padding: 1rem;
}
.grid__item {
  outline: 4px solid #111;
  background: #557;
  color: #fff;
  font-size: 1.5rem;
  text-shadow: 1px 1px 0 rgb(0 0 0 / 0.8);
}

.subgrid__item,
.nested__item {
  outline: 2px solid lime;
}

body {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 10px;
}

.grid__container {
  display: grid;
  min-width: 40vw;
  grid-template-columns: repeat(4, [col-start] 1fr [col-end]);
  grid-template-rows: repeat(2, [row-start] minmax(100px, auto) [row-end]);
  grid-template-areas:
    "cell1 cell2 cell3 cell4"
    "cell5 cell6 cell7 cell8";
  gap: 1rem;
}

.grid__container--nested,
.grid__container--subgrid {
  grid-column: 2 / 5;
  grid-row: 1 / 2;
  display: inherit;
  gap: 20px;
  padding: 0.5rem;
  outline: 4px solid #ccc;
}

.grid__container--nested {
  grid-template-columns: inherit;
  grid-template-rows: inherit;
}

.grid__container--subgrid {
  outline: 4px solid #f36;
  grid-template-rows: subgrid;
  grid-template-columns: subgrid;
}