Emulate mobile devices in Playwright and Puppeteer using built-in device descriptors and understand their limits

Steps

1. In Playwright, import `devices` from `@playwright/test` and pass a device descriptor to the browser context: `browser.newContext({ ...devices['iPhone 14'], locale: 'en-US' })` — this sets viewport size, user agent, device scale factor, and touch support at once.
2. In Puppeteer, use `page.emulate(puppeteer.KnownDevices['iPhone 14'])` which sets the viewport and user agent from the bundled device list.
3. Override individual properties after applying the descriptor when you need a custom size: set `viewport` explicitly or call `page.setViewportSize` after the context is created.
4. Test touch interactions by using Playwright's `page.touchscreen.tap(x, y)` or trigger pointer events; desktop mouse events are not automatically converted to touch events by emulation.
5. Test network throttling by combining device emulation with a Chrome DevTools Protocol `Network.emulateNetworkConditions` call (available via `page.context().newCDPSession(page)` in Playwright) to simulate slow connections.