Fix things up.

packages/block-library/src/embed/style.scss

@@ -14,4 +14,59 @@
 	figcaption {
 		@include caption-style();
 	}
+
+	// Add responsiveness to common aspect ratios.
+	&.wp-embed-aspect-21-9 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-18-9 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-16-9 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-4-3 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-1-1 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-9-16 .wp-block-embed__wrapper,
+	&.wp-embed-aspect-1-2 .wp-block-embed__wrapper {
+		position: relative;
+
+		&::before {
+			content: "";
+			display: block;
+			padding-top: 50%; // Default to 2:1 aspect ratio.
+		}
+
+		iframe {
+			position: absolute;
+			top: 0;
+			right: 0;
+			bottom: 0;
+			left: 0;
+			width: 100%;
+			height: 100%;
+		}
+	}
+
+	&.wp-embed-aspect-21-9 .wp-block-embed__wrapper::before {
+		padding-top: 42.85%; // 9 / 21 * 100
+	}
+
+	&.wp-embed-aspect-18-9 .wp-block-embed__wrapper::before {
+		padding-top: 50%; // 9 / 18 * 100
+	}
+
+	&.wp-embed-aspect-16-9 .wp-block-embed__wrapper::before {
+		padding-top: 56.25%; // 9 / 16 * 100
+	}
+
+	&.wp-embed-aspect-4-3 .wp-block-embed__wrapper::before {
+		padding-top: 75%; // 3 / 4 * 100
+	}
+
+	&.wp-embed-aspect-1-1 .wp-block-embed__wrapper::before {
+		padding-top: 100%; // 1 / 1 * 100
+	}
+
+	&.wp-embed-aspect-9-6 .wp-block-embed__wrapper::before {
+		padding-top: 66.66%; // 6 / 9 * 100
+	}
+
+	&.wp-embed-aspect-1-2 .wp-block-embed__wrapper::before {
+		padding-top: 200%; // 2 / 1 * 100
+	}
 }

packages/block-library/src/theme.scss

@@ -5,4 +5,3 @@
 @import "./separator/theme.scss";
 @import "./table/theme.scss";
 @import "./video/theme.scss";
-@import "./embed/theme.scss";