A headless browser is a web browser without Graphical User Interface (GUI). It accesses and renders web pages but doesn't show them to any human being. A headless browser should be able to parse JavaScript. Currently, most of the systems encourage tests against headless browsers due to their efficiency and time-saving properties. PhantomJS and HTMLUnit are the most commonly-used headless browsers. Capybara-webkit is another efficient headless WebKit for rails-based applications.
PhantomJS is a headless WebKit scriptable with the JavaScript API. It is generally used for the headless testing of web applications that come with GhostDriver built-in. Tests on PhantomJS are obviously fast since it has fast and native support for various web standards, such as DOM handling, CSS selector, JSON, canvas, and SVG. In general, WebKit is a layout engine that allows web browsers to render web pages. Some browsers, such as Safari and Chrome, use WebKit.
Apparently...