SauceLabs (commercial) allows you to run unit tests inside of an emulator, which can be really useful for scripting a flow through your site and watch the video recording of this afterwards on various devices.You can also fire up multiple emulators in the same screen, letting you test how your app looks and feels across multiple devices at the same time. You select an operating system, select your browser version and device type, select a URL to browse, and it spins up a hosted virtual machine that you can interact with. BrowserStack (commercial) is the easiest to use for manual testing.A big advantage of cloud-based emulators over real devices and local emulators is that you can automate unit tests for your site across different platforms. If you can’t use the emulators and don't have access to real devices, then cloud-based emulators are the next best thing.