I think knowing HTML, Web Development tools, CSS, etc. can make you a good Website Developer, but not neccessarily a good Webmaster.

A Webmaster has to manage a whole website, not necessarily program it from the bottom-up. The webmaster should know each and every thing about his servers on which the site is hosted. Sharp knowledge of HTML, CSS, JavaScript, and server-side scripts is required so that he can make quick changes to the website himself. He should know the ins- and outs- of disaster management when something goes wrong with the server/site.

A webmaster should also be savvy with security discussions, as he should know exactly when and how his site is being attacked upon, if somehow.