diff --git a/.changeset/hip-goats-smoke.md b/.changeset/hip-goats-smoke.md
new file mode 100644
index 0000000000..ce023460b7
--- /dev/null
+++ b/.changeset/hip-goats-smoke.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure custom element styles append correctly during prod
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 82d49f0ad7..6ebb77f2e9 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -380,7 +380,9 @@ export function client_component(analysis, options) {
 		state.hoisted.push(b.const('$$css', b.object([b.init('hash', hash), b.init('code', code)])));
 
 		component_block.body.unshift(
-			b.stmt(b.call('$.append_styles', b.id('$$anchor'), b.id('$$css')))
+			b.stmt(
+				b.call('$.append_styles', b.id('$$anchor'), b.id('$$css'), options.customElement && b.true)
+			)
 		);
 	}
 
diff --git a/packages/svelte/src/internal/client/dom/css.js b/packages/svelte/src/internal/client/dom/css.js
index 1cccb42b3e..dbf862543f 100644
--- a/packages/svelte/src/internal/client/dom/css.js
+++ b/packages/svelte/src/internal/client/dom/css.js
@@ -6,10 +6,11 @@ var seen = new Set();
 /**
  * @param {Node} anchor
  * @param {{ hash: string, code: string }} css
+ * @param {boolean} [is_custom_element]
  */
-export function append_styles(anchor, css) {
+export function append_styles(anchor, css, is_custom_element = false) {
 	// in dev, always check the DOM, so that styles can be replaced with HMR
-	if (!DEV) {
+	if (!DEV && !is_custom_element) {
 		if (seen.has(css)) return;
 		seen.add(css);
 	}