diff --git a/.changeset/cuddly-camels-hang.md b/.changeset/cuddly-camels-hang.md
new file mode 100644
index 000000000000..478598f3444d
--- /dev/null
+++ b/.changeset/cuddly-camels-hang.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Warn if comments are removed from HTML
diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index 030b435613f7..851c1b04c4ca 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -6,6 +6,7 @@ import { s } from '../../../utils/misc.js';
 import { Csp } from './csp.js';
 import { uneval_action_response } from './actions.js';
 import { clarify_devalue_error } from '../utils.js';
+import { DEV } from 'esm-env';
 
 // TODO rename this function/module
 
@@ -353,17 +354,34 @@ export async function render_response({
 	// add the content after the script/css links so the link elements are parsed first
 	head += rendered.head;
 
+	const html = options.app_template({
+		head,
+		body,
+		assets,
+		nonce: /** @type {string} */ (csp.nonce)
+	});
+
 	// TODO flush chunks as early as we can
-	const html =
+	const transformed =
 		(await resolve_opts.transformPageChunk({
-			html: options.app_template({ head, body, assets, nonce: /** @type {string} */ (csp.nonce) }),
+			html,
 			done: true
 		})) || '';
 
+	if (DEV && page_config.csr) {
+		if (transformed.split('<!--').length < html.split('<!--').length) {
+			// the \u001B stuff is ANSI codes, so that we don't need to add a library to the runtime
+			// https://svelte.dev/repl/1b3f49696f0c44c881c34587f2537aa2
+			console.warn(
+				"\u001B[1m\u001B[31mRemoving comments in transformPageChunk can break Svelte's hydration\u001B[39m\u001B[22m"
+			);
+		}
+	}
+
 	const headers = new Headers({
 		'x-sveltekit-page': 'true',
 		'content-type': 'text/html',
-		etag: `"${hash(html)}"`
+		etag: `"${hash(transformed)}"`
 	});
 
 	if (!state.prerendering) {
@@ -381,7 +399,7 @@ export async function render_response({
 		}
 	}
 
-	return new Response(html, {
+	return new Response(transformed, {
 		status,
 		headers
 	});