Fix the placeholder attribute and caption placeholder color

packages/base-styles/_mixins.scss

@@ -278,6 +278,10 @@
 		@include input-style__focus();
 	}
 
+	@include input-placeholder-color;
+}
+
+@mixin input-placeholder-color {
 	// Use opacity to work in various editor styles.
 	&::-webkit-input-placeholder {
 		color: $dark-gray-placeholder;

packages/block-editor/src/components/rich-text/content.scss

@@ -5,9 +5,15 @@
 
 	[data-rich-text-placeholder]::after {
 		content: attr(data-rich-text-placeholder);
-		// Use opacity to work in various editor styles.
-		// We don't specify the color here, because blocks or editor styles might provide their own.
-		opacity: 0.62;
+		// The RichText placeholder is the equivalent of the HTML placeholder
+		// attribute. As such, its color should have a sufficient contrast ratio
+		// with the background. However, themes, blocks or editor styles might
+		// provide their own color for the text of the block where RichText is
+		// used. For this reason, the RichText placeholder color should resemble
+		// the text color. We assume the block text color is responsible for
+		// providing sufficient contrast and slightly dim the placeholder color.
+		color: currentColor;
+		opacity: 85%;
 	}
 
 	&:focus {
@@ -16,12 +22,6 @@
 	}
 }
 
-// Captions may have lighter (gray) text, or be shown on a range of different background luminosites.
-// To ensure legibility, we increase the default placeholder opacity to ensure contrast.
-figcaption.block-editor-rich-text__editable [data-rich-text-placeholder]::before {
-	opacity: 0.8;
-}
-
 [data-rich-text-script] {
 	display: inline;
 

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

@@ -34,6 +34,15 @@
 		background-color: $black;
 	}
 
+	// Make sure the placeholder text is white when has-background-dim default background is black.
+	// the first selector is required for old Cover markup
+	&.has-background-dim:not([class*="-background-color"]):not(.is-light),
+	&:not(.is-light) .has-background-dim:not([class*="-background-color"]) {
+		& ~ .wp-block-cover__inner-container [data-rich-text-placeholder]::after {
+			color: $white;
+		}
+	}
+
 	.has-background-dim.has-background-gradient {
 		background-color: transparent;
 	}

packages/editor/src/components/post-text-editor/style.scss

@@ -35,15 +35,5 @@ textarea.editor-post-text-editor {
 		position: relative;
 	}
 
-	&::-webkit-input-placeholder {
-		color: $dark-gray-placeholder;
-	}
-
-	&::-moz-placeholder {
-		color: $dark-gray-placeholder;
-	}
-
-	&:-ms-input-placeholder {
-		color: $dark-gray-placeholder;
-	}
+	@include input-placeholder-color;
 }