[johnmurch]

Someone who can do backend development (node.js, ruby, php, whatever) as well as front-end development (css/html/javascript/ember/angular/backbone)

[NikolaTesla]

Is it enough to know one of for the backend and frontend or do you have to have a broader list of expertise?

[johnmurch]

You should have a broad range, so you can fix css issues, javascript frontend issues as well as build a new feature that integrates with a database.

As shown above, but basically you can code a web app from start to finish (e.g. backend code/framework like ruby on rails, frontend like css/html/etc and integrate some time of database (e.g. MySQL or Mongo).