Off the top of my head: Data types - Both concrete classes or helpers for manipulating data. Input Validation - Do it client-side to do it fast, but double ch

Off the top of my head: Data types - Both concrete classes or helpers for manipulating data. Input Validation - Do it client-side to do it fast, but double check on the server. Formatting - Larger web apps often need to format things from dates to users to numbers on client-side generated data without the server hop. Templates - Many templating systems are compatible with multiple languages, but its easier if there's just one output type.