Composition II in Red, Blue, and Yellow

<div class="canvas">
  <div class="rect1 left top"></div>
  <div class="rect2 top right"></div>
  <div class="rect3 left"></div>
  <div class="rect4 left bottom"></div>
  <div class="rect5 bottom"></div>
  <div class="rect6 right"></div>
  <div class="rect7 right bottom"></div>
</div>

.canvas {
  width: 100%;
  max-width: 600px;
  aspect-ratio: 1;
  padding-inline: 0.5rem;

  display: grid;
  grid-template-columns: 25% 65% 10%;
  grid-template-rows: 15% 60% 12.5% 12.5%;

  background: white;
}

.canvas div {
  border: 5px solid black;
  box-shadow: rgba(0, 0, 0, 0.5) 0 1px 5px 2px;
}

div.left {
  border-left: none;
}

div.top {
  border-top: none;
}

div.right {
  border-right: none;
}

div.bottom {
  border-bottom: none;
}

.rect2 {
  grid-column: 2/ -1;
  grid-row: 1 / span 2;
  background: red;
}

div.rect3 {
  border-top: 10px solid black;
}

.rect4 {
  background: blue;
  grid-row: 3 / span 2;
}

.rect5 {
  grid-row: 3 / span 2;
}

div.rect7 {
  background: yellow;
  border-top: 10px solid black;
}

/* general */

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  height: 100dvh;

  display: grid;
  place-items: center;
}