only add third argument for custom elements

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -381,7 +381,7 @@ export function client_component(analysis, options) {
 
 		component_block.body.unshift(
 			b.stmt(
-				b.call('$.append_styles', b.id('$$anchor'), b.id('$$css'), b.literal(options.customElement))
+				b.call('$.append_styles', b.id('$$anchor'), b.id('$$css'), options.customElement && b.true)
 			)
 		);
 	}

packages/svelte/src/internal/client/dom/css.js

@@ -6,9 +6,9 @@ var seen = new Set();
 /**
  * @param {Node} anchor
  * @param {{ hash: string, code: string }} css
- * @param {boolean} is_custom_element
+ * @param {boolean} [is_custom_element]
  */
-export function append_styles(anchor, css, is_custom_element) {
+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 && !is_custom_element) {
 		if (seen.has(css)) return;