Questions regarding usage of headless browsers for automation of testing web applications

Headless browsers provide automated control of a web page in an environment similar to popular web browsers, but are executed via a command line interface or using network communication. They are particularly useful for testing web pages as they are able to render and understand HTML the same way a browser would, including styling elements such as page layout, colour, font selection and execution of JavaScript and AJAX which are usually not available when using other testing methods.