Why would you need expertise in various industries to work on a website? I've done websites for all different types of businesses.
Where you would specialize is in the technology you use. If you want to specialize in an open-source platform (Joomla, Drupal, Wordpress, etc.), if you want to specialize in frontend coding (CSS + Jquery or Mootools), etc.
It's usually difficult to be a php programmer and a frontend code, althought I've seen it. So you specializing in frontend coding and hire someone to do the backend coding or vice versa.