preserve headers per the spec

packages/kit/src/runtime/server/index.js

@@ -129,11 +129,28 @@ export async function respond(incoming, options, state = {}) {
 								const etag = `"${hash(response.body || '')}"`;
 
 								if (if_none_match_value === etag) {
+									/** @type {import('types/helper').ResponseHeaders} */
+									const headers = { etag };
+
+									// https://datatracker.ietf.org/doc/html/rfc7232#section-4.1
+									for (const key of [
+										'cache-control',
+										'content-location',
+										'date',
+										'expires',
+										'vary'
+									]) {
+										if (key in response.headers) {
+											headers[key] = /** @type {string} */ (response.headers[key]);
+										}
+									}
+
 									return {
 										status: 304,
-										headers: { etag }
+										headers
 									};
 								}
+
 								response.headers['etag'] = etag;
 							}
 						}

packages/kit/test/apps/basics/src/routes/etag/text.js

@@ -1,6 +1,9 @@
 /** @type {import('@sveltejs/kit').RequestHandler} */
 export function get() {
 	return {
-		body: 'some text'
+		body: 'some text',
+		headers: {
+			expires: 'yesterday'
+		}
 	};
 }

packages/kit/test/apps/basics/test/test.js

@@ -642,6 +642,7 @@ test.describe.parallel('ETags', () => {
 		});
 
 		expect(r2.status()).toBe(304);
+		expect(r2.headers()['expires']).toBe('yesterday');
 	});
 
 	test('generates etag/304 for binary body', async ({ request }) => {