I like my exceptional pages (404, 500, 503) to rely as little as possible on the webserver to render in the browser. A nice tool to support this is CSS Used for Chrome.
CSS Used adds a panel to DevTools that reports which CSS classes from external stylesheets are being used on your page. You can then paste these styles inside a
style tag and remove stylesheet references in your HTML. I iterate on this by opening the HTML file in my browser outside of my project repo; when all the styles are loading, I’ve removed the stylesheet dependency.