We do not need to open up an instance of Browser(firefox/chrome/IE) to execute our Selenium tests
Having the tests run in a browser allows us to view the execution as it is happening, which helps in the writing and debugging stages of a given test. However, once the test is complete and is reliably running in CI(Continuous Integration), we don’t always need to physically watch it run. A headless browser is a solution for these scenarios.
Using a headless browser, such as HTMLUnit Driver, increases the execution speed of the test suite, since the browser is not required to render the whole DOM. Furthermore, using a headless browser is a lot less resource-intensive, allowing a higher quantity of parallel-running tests.