Web Test Runner is a package within the Modern Web project, which provides several high-quality tools for modern web development with support for web standards like ES Modules. Web Test Runner is a test runner for web applications. One of its advantages compared to existing test runners is that it runs tests in the browser (which could be headless). It supports multiple browser launchers — including Puppeteer, Playwright, and Selenium — and uses Mocha by default for the test framework. The tests run pretty fast, and we like that we can open a browser window with devtools when debugging. Web Test Runner internally uses Web Dev Server which allows us to leverage its great plugin API for adding customized plugins for our test suite. Modern Web tools look like a very promising developer toolchain, and we're already using it in a few projects.