update style_manager to use customStyleTag

src/runtime/internal/dom.ts

@@ -18,6 +18,9 @@ export function append_styles(
 	}
 }
 
+export function append_empty_stylesheet() {
+	return appendStylesTo.appendChild(element('style') as HTMLStyleElement);
+}
 
 export function append(target: Node, node: Node) {
 	target.appendChild(node);

src/runtime/internal/style_manager.ts

@@ -1,4 +1,4 @@
-import { element } from './dom';
+import { append_empty_stylesheet } from './dom';
 import { raf } from './environment';
 
 interface ExtendedDoc extends Document {
@@ -31,7 +31,7 @@ export function create_rule(node: Element & ElementCSSInlineStyle, a: number, b:
 	const name = `__svelte_${hash(rule)}_${uid}`;
 	const doc = node.ownerDocument as ExtendedDoc;
 	active_docs.add(doc);
-	const stylesheet = doc.__svelte_stylesheet || (doc.__svelte_stylesheet = doc.head.appendChild(element('style') as HTMLStyleElement).sheet as CSSStyleSheet);
+	const stylesheet = doc.__svelte_stylesheet || (doc.__svelte_stylesheet = append_empty_stylesheet().sheet as CSSStyleSheet);
 	const current_rules = doc.__svelte_rules || (doc.__svelte_rules = {});
 
 	if (!current_rules[name]) {