Gecko is not an HTML rendering engine, it's a Mozilla XUL+HTML rendering engine. It's a legacy codebase and not easily embeddable. XULRunner and other developm

Gecko is not an HTML rendering engine, it's a Mozilla XUL+HTML rendering engine. It's a legacy codebase and not easily embeddable. XULRunner and other developments are changing that but that still involves XUL. I'd love for a separate pure-HTML Gecko engine that competes with Webkit, but until that happens, there are technical reasons for choosing one or the either.