It depends upon your skills. I don't think HTML(or XHTML) will pay enough. Learn at least one client-side scripting language, one server-side scripting and one database support MySQL. You already know an desktop application language.
These 4 basics should be enough to get you started.
And yeah, for web designing, you may need to sharpen your skill learning AJAX, action script, graphics design, etc. Else you may have to sub-contract it and pay heavily for it.