Why would you say that? I would say that the most important web development tool is HTML. Without it, we wouldn't have a web at all. However, I don't look at any programming or markup language as a 'tool' just as I don't look at spoken language as a tool, although perhaps all are in a very broad sense.
I also wouldn't add proprietary WYSIWYG's as important tools to Web Development, especially since the advent of many, many free web development platforms such as WordPress, Joomla, ModX and others.