Depends on if you're gonna freelance or work for a webdesign house. If you work for yourself, you might need to know a little of everything to successfully design a website from start to finish. Working for a webdesign company usually means tasks are broken up into sections. They may have a Graphics section, html/CSS section, programming section.
.dotnet, Microsoft servers, such a very small %age of clients possible, but many of those clients are large and require large maintainence contracts, because the type of product they are, but the larger they are the more likely they are locked into maintainence agreements, proprietary codebase -more difficult to outsource
java a slightly larger %age of customers
Apache *n?x (Linux Unix PosIx all the possible combinations) php RoR a very much larger %age client base, often of smaller clients who each require smaller amounts of maintainence
large resource availablility, can integrate into finished work
sql, as mssql mysql sqllite PostgreSql needed for all
.dotnet and java alone, unless you are real good, and real lucky, starvation status
I think Bob is right. Google "servers" for popularity, as well as server side languages such as PHP and Coldfusion and see how much of the market utilizes these. Most medium to large size companies utilize PHP or CF and databses such as MSSQL and MySQL.