preact-custom-element/web-test-runner.config.mjs

Wrap your component up as a custom element

import { esbuildPlugin } from '@web/dev-server-esbuild';

export default {
	nodeResolve: true,
	plugins: [
		esbuildPlugin({
			jsx: true,
			jsxFactory: 'h',
			jsxFragment: 'Fragment',
		}),
	],
};