E2E testing using a headless browser controlled by a framework like puppeteer. Scaling up with many different OS/browser combinations gets difficult and expensi

E2E testing using a headless browser controlled by a framework like puppeteer. Scaling up with many different OS/browser combinations gets difficult and expensive fast though, which is why services like browserstack exist. Testing CSS is sometimes done by diffing screenshots but I personally don't think it's worth the effort unless your app has hundreds of pages.