diff --git a/package.json b/package.json
index 09df48cbe067..ceac177e805e 100644
--- a/package.json
+++ b/package.json
@@ -125,6 +125,7 @@
     "@types/yargs-parser": "^20.2.1",
     "chai": "^4.3.4",
     "cheerio": "^1.0.0-rc.10",
+    "hast-util-select": "^5.0.1",
     "mocha": "^9.1.3",
     "vite": "~2.6.10"
   },
diff --git a/test/astro-markdown-plugins.test.js b/test/astro-markdown-plugins.test.js
index aca9fcf7b1fe..ee780a738ef5 100644
--- a/test/astro-markdown-plugins.test.js
+++ b/test/astro-markdown-plugins.test.js
@@ -2,6 +2,7 @@ import { expect } from 'chai';
 import cheerio from 'cheerio';
 import { loadFixture } from './test-utils.js';
 import markdownRemark from '@astrojs/markdown-remark';
+import addClasses from './fixtures/astro-markdown-plugins/add-classes.mjs';
 
 describe('Astro Markdown plugins', () => {
 	let fixture;
@@ -15,11 +16,7 @@ describe('Astro Markdown plugins', () => {
 					markdownRemark,
 					{
 						remarkPlugins: ['remark-code-titles', ['rehype-autolink-headings', { behavior: 'prepend' }]],
-						rehypePlugins: [
-							[import('rehype-toc'), { headings: ['h2', 'h3'] }],
-							[import('../../../examples/with-markdown-plugins/add-classes.mjs'), { 'h1,h2,h3': 'title' }],
-							'rehype-slug',
-						],
+						rehypePlugins: [['rehype-toc', { headings: ['h2', 'h3'] }], [addClasses, { 'h1,h2,h3': 'title' }], 'rehype-slug'],
 					},
 				],
 			},
diff --git a/test/fixtures/astro-markdown-plugins/add-classes.mjs b/test/fixtures/astro-markdown-plugins/add-classes.mjs
new file mode 100644
index 000000000000..39acabd52670
--- /dev/null
+++ b/test/fixtures/astro-markdown-plugins/add-classes.mjs
@@ -0,0 +1,18 @@
+import { selectAll } from 'hast-util-select';
+
+export default (additions) => {
+	const adders = Object.entries(additions).map(adder);
+	return (node) => adders.forEach((a) => a(node));
+};
+
+const adder = ([selector, className]) => {
+	const writer = write(className);
+	return (node) => selectAll(selector, node).forEach(writer);
+};
+
+const write =
+	(className) =>
+	({ properties }) => {
+		if (!properties.className) properties.className = className;
+		else properties.className += ` ${className}`;
+	};