Debugging Web applications is difficult because of their distributed nature but also because of the programming languages and tools commonly used to develop them. Taking benefit of the multitier aspect of the Hop programming language, we have built a new debugger for Web applications that copes with the server-side and the client-side of the executions. Its advantage over most debuggers for the Web is that it reports the full stack trace containing all the server-side and client-side frames that have conducted to an error. An error is reported on its actual position on the source code, wherever it occurs on the server or on the client. To help detecting errors as early as possible, the Hop debugger is accompanied with a debugging execution mode where types are checked before data structures are accessed, argument numbers are verified before functions are called, and array bounds are checked before vectors are accessed. Combining the debugger and the debugging mode makes errors of Web applications easier to understand and easier to localize. Hopefully they also become easier to fix.