In order to write a webapp, you need to know (in order of importance): (X)HTML, any or all of Perl/PHP/Ruby/Python, SQL, CSS, Javascript.

If you want to be employed, you'll need to know all of the above. If you're a part of a larger team you might be able to skip out on CSS and Javascript or Perl/PHP/Ruby/Python and SQL. If you have to choose, skip on the former; it's the most unpleasant part of webapp development.