Index: src/wp-includes/js/wp-emoji-loader.js
===================================================================
--- src/wp-includes/js/wp-emoji-loader.js	(revision 38190)
+++ src/wp-includes/js/wp-emoji-loader.js	(working copy)
@@ -14,7 +14,7 @@
 		var canvas = document.createElement( 'canvas' ),
 			context = canvas.getContext && canvas.getContext( '2d' ),
 			stringFromCharCode = String.fromCharCode,
-			tonedata, tone, tone2;
+			flag, flag2, tonedata, tone, tone2;
 
 		if ( ! context || ! context.fillText ) {
 			return false;
@@ -40,7 +40,29 @@
 				 * to a larger image (4-5KB data URL).
 				 */
 				context.fillText( stringFromCharCode( 55356, 56806, 55356, 56826 ), 0, 0 );
-				return canvas.toDataURL().length > 3000;
+				if ( canvas.toDataURL().length < 3000 ) {
+					return false;
+				}
+
+				context.clearRect( 0, 0, canvas.width, canvas.height );
+
+				/*
+				 * Test for rainbow flag compatibility. As the rainbow flag was added out of sequence with
+				 * the usual Unicode release cycle, some browsers support it, and some don't, even if their
+				 * Unicode support is up to date.
+				 *
+				 * To test for support, we try to render it, and compare the rendering to how it would look if
+				 * the browser doesn't render it correctly (white flag emoji + rainbow emoji).
+				 */
+				context.fillText( stringFromCharCode( 55356, 57331, 65039, 8205, 55356, 57096 ), 0, 0 );
+				flag = canvas.toDataURL();
+
+				context.clearRect( 0, 0, canvas.width, canvas.height );
+
+				context.fillText( stringFromCharCode( 55356, 57331, 55356, 57096 ), 0, 0 );
+				flag2 = canvas.toDataURL();
+
+				return flag !== flag2;
 			case 'diversity':
 				/*
 				 * This tests if the browser supports the Emoji Diversity specification, by rendering an
Index: src/wp-includes/js/wp-emoji.js
===================================================================
--- src/wp-includes/js/wp-emoji.js	(revision 38190)
+++ src/wp-includes/js/wp-emoji.js	(working copy)
@@ -180,8 +180,10 @@
 							return false;
 					}
 
-					if ( settings.supports.everythingExceptFlag && ! /^1f1(?:e[6-9a-f]|f[0-9a-f])-1f1(?:e[6-9a-f]|f[0-9a-f])$/.test( icon ) ) {
-
+					if ( settings.supports.everythingExceptFlag
+						&& ! /^1f1(?:e[6-9a-f]|f[0-9a-f])-1f1(?:e[6-9a-f]|f[0-9a-f])$/.test( icon ) // Country flags
+						&& ! /^(1f3f3-fe0f-200d-1f308|1f3f4-200d-2620-fe0f)$/.test( icon )          // Rainbow and pirate flags
+					) {
 						return false;
 					}