core-bundle/src/EventListener/PrettyErrorScreenListener.php
public function onKernelException(GetResponseForExceptionEvent $event): void
    {
        if (!$event->isMasterRequest()) {
            return;
        }

        $request = $event->getRequest();

        if ('html' !== $request->getRequestFormat()) {
            return;
        }

        if (!AcceptHeader::fromString($request->headers->get('Accept'))->has('text/html')) {
            return;
        }

        $this->handleException($event);
    }
public function onKernelException(GetResponseForExceptionEvent $event): void
    {
        if (!$event->isMasterRequest()) {
            return;
        }

        $request = $event->getRequest();

        if ('html' !== $request->getRequestFormat()) {
            return;
        }

        if (!AcceptHeader::fromString($request->headers->get('Accept'))->has('text/html')) {
            return;
        }

        $this->handleException($event);
    }
Map an exception to an error screen.