HTTP Request-Based Testing Does Not Test the UI

It is very important to keep in mind that a web test simulates the HTTP request traffic that is generated
when a user exercises your web site. It does not exercise any of the UI elements or JavaScript on
your pages. As such, it does not test the UI that your users see, but it does test the core functionality of
your system. Often with a web site the amount of functionality that is in the client UI is pretty minimal,
but it can be an issue if much of your site has script-heavy pages or uses AJAX.
This is good for a couple of reasons. It means that your tests are not affected by UI changes. UI-based
testing is often fragile because the UI changes more frequently than the underlying HTTP requests being
used. The other benefit is that UI tests cannot effectively be used for load testing. Having the web tests
not be UI-based means that when they are included in a load test, you can execute multiple instances of
the test at the same time, allowing simulating 100 users, for example, all executing the same web test on
a single computer.
But the drawback of using HTTP requests is that your UI and the scripts on the page do not get exercised.
This has two main effects that you need to watch out for: inability to properly test your UI and scripts
affecting your dynamic data.