.section {
    position: relative;
    display: block;
    width: 100vw;
    height: 100vh;
    height: 100vh; /* Use vh as a fallback for browsers that do not support Custom Properties */
    height: calc(var(--vh, 1vh) * 100);
    background: yellow;
}

.intro {
    background: blue;
}
.intro-canvas {
    width: 100%;
    height: 100%;
    height: 100vh; /* Use vh as a fallback for browsers that do not support Custom Properties */
    height: calc(var(--vh, 1vh) * 100);
}