diff --git a/cli.js b/cli.js
index 33928af..11255ac 100755
--- a/cli.js
+++ b/cli.js
@@ -24,6 +24,7 @@ const cli = meow(
     --only-style           Only output the styles, forces --preserve-variables on
     --only-variables       Only output the variables for the specified themes
     --root-selector        Specify the root selector when outputting styles, default '.markdown-body'
+    --no-use-fixture       Exclude generated classes that come from GitHub Markdown API rendered fixture.md
 
   Examples
     $ github-markdown-css --list
@@ -74,6 +75,10 @@ const cli = meow(
 			rootSelector: {
 				type: 'string',
 			},
+			useFixture: {
+				type: 'boolean',
+				default: true,
+			},
 		},
 	},
 );
@@ -85,6 +90,7 @@ const {
 	onlyStyle,
 	onlyVariables,
 	rootSelector,
+	useFixture,
 } = cli.flags;
 
 let {light, dark} = cli.flags;
@@ -140,5 +146,6 @@ console.log(
 		onlyStyles: onlyStyle,
 		onlyVariables,
 		rootSelector,
+		useFixture,
 	}),
 );
diff --git a/index.js b/index.js
index ddae818..f863408 100644
--- a/index.js
+++ b/index.js
@@ -1,8 +1,10 @@
 import postcss from 'postcss';
-import {cachedFetch, reverseUnique, unique, zip} from './utilities.js';
+import {cachedFetch, getUniqueClasses, renderMarkdown, reverseUnique, unique, zip} from './utilities.js';
 import {ALLOW_CLASS, ALLOW_TAGS, manuallyAddedStyle} from './constants.js';
 
-function extractStyles(rules, cssText) {
+function extractStyles(rules, cssText, {extraAllowableClasses = []} = {}) {
+	const allowableClassList = new Set([...ALLOW_CLASS, ...extraAllowableClasses]);
+
 	function select(selector) {
 		if (selector.startsWith('.markdown-body')) {
 			return true;
@@ -23,7 +25,7 @@ function extractStyles(rules, cssText) {
 			}
 
 			const klass = selector.match(/\.[-\w]+/);
-			if (klass && !ALLOW_CLASS.has(klass[0])) {
+			if (klass && !allowableClassList.has(klass[0])) {
 				return false;
 			}
 
@@ -32,7 +34,7 @@ function extractStyles(rules, cssText) {
 
 		const klass = selector.match(/^\.[-\w]+/);
 		if (klass) {
-			return ALLOW_CLASS.has(klass[0]);
+			return allowableClassList.has(klass[0]);
 		}
 
 		return false;
@@ -79,7 +81,7 @@ function extractStyles(rules, cssText) {
 			return;
 		}
 
-		if (rule.some(decl => decl.value.includes('prettylights'))) {
+		if (rule.some(node => node.type === 'decl' && node.value.includes('prettylights'))) {
 			if (!rule.selector.includes('.QueryBuilder')) {
 				rules.push(rule);
 			}
@@ -135,6 +137,10 @@ function classifyRules(rules) {
 		return undefined;
 	}
 
+	function isEmpty(rule) {
+		return !rule.first;
+	}
+
 	function mergeRules(rules) {
 		const result = [];
 		const selectorIndexMap = {};
@@ -163,7 +169,7 @@ function classifyRules(rules) {
 			});
 		}
 
-		return result;
+		return result.filter(rule => !isEmpty(rule));
 	}
 
 	const result = {rules: [], light: [], dark: []};
@@ -233,6 +239,7 @@ export default async function getCSS({
 	onlyVariables = false,
 	onlyStyles = false,
 	rootSelector = '.markdown-body',
+	useFixture = true,
 } = {}) {
 	if (onlyVariables && onlyStyles) {
 		// Would result in an empty output
@@ -250,7 +257,12 @@ export default async function getCSS({
 	const body = await cachedFetch('https://github.com');
 	// Get a list of all css links on the page
 	const links = unique(body.match(/(?<=href=").+?\.css/g));
-	const contents = await Promise.all(links.map(url => cachedFetch(url)));
+	const renderMarkdownPromise = useFixture ? renderMarkdown() : Promise.resolve();
+	const [fixtureHtml, ...contents] = await Promise.all([
+		renderMarkdownPromise,
+		...links.map(url => cachedFetch(url)),
+	]);
+	const fixtureClasses = getUniqueClasses(fixtureHtml);
 
 	let rules = [];
 	const colors = [];
@@ -277,7 +289,7 @@ export default async function getCSS({
 				extractVariables(colors, 'shared', cssText);
 			}
 
-			extractStyles(rules, cssText);
+			extractStyles(rules, cssText, {extraAllowableClasses: fixtureClasses});
 		}
 	}
 
diff --git a/utilities.js b/utilities.js
index 89782af..c333a25 100644
--- a/utilities.js
+++ b/utilities.js
@@ -75,7 +75,11 @@ export async function renderMarkdown() {
 
 	const response = await fetch('https://api.github.com/markdown', {
 		method: 'POST',
-		body: JSON.stringify({text}),
+		body: JSON.stringify({
+			text,
+			mode: 'gfm',
+			context: 'sindresorhus/generate-github-markdown-css',
+		}),
 		headers: {
 			Accept: 'application/vnd.github.v3+json',
 			'User-Agent': 'Node.js',
@@ -91,3 +95,9 @@ export async function renderMarkdown() {
 
 	throw new Error(`Failed to render markdown: ${body}`);
 }
+
+export function getUniqueClasses(html = '') {
+	const classNames = [...html.matchAll(/class\s*=\s*["']([^"']+)["']/g)]
+		.flatMap(match => match[1].split(/\s+/).map(c => `.${c}`));
+	return new Set(classNames);
+}