With sufficient sandboxing, and depending on the needs of the site in question, this is actually possible to do securely. When you use sandboxing properly, the

With sufficient sandboxing, and depending on the needs of the site in question, this is actually possible to do securely. When you use sandboxing properly, the security of your application doesn't depend on how complex it is or how many features it has. This is a huge advantage over secure programming languages, where bugs are less likely but still some constant factor of KLOC. That said, not every application can be sandboxed easily.