only add markers to head elements in SSR with hydratable=true (#4258)

src/compiler/compile/render_ssr/handlers/Element.ts

@@ -124,7 +124,7 @@ export default function(node: Element, renderer: Renderer, options: RenderOption
 		}
 	});
 
-	if (options.head_id) {
+	if (options.hydratable && options.head_id) {
 		renderer.add_string(` data-svelte="${options.head_id}"`);
 	}
 

src/compiler/compile/render_ssr/handlers/Title.ts

@@ -5,7 +5,11 @@ import { x } from 'code-red';
 export default function(node: Title, renderer: Renderer, options: RenderOptions) {
 	renderer.push();
 
-	renderer.add_string(`<title data-svelte="${options.head_id}">`);
+	renderer.add_string('<title');
+	if (options.hydratable && options.head_id) {
+		renderer.add_string(` data-svelte="${options.head_id}"`);
+	}
+	renderer.add_string('>');
 
 	renderer.render(node.children, options);