Why only web? Modal dialogs should be forbidden everywhere.
Modal dialogue boxes are for lazy programming not for UI. Almost nothing in real life is modal. Don't break a user's flow! We just built a full application platform (i.e. dev kit, apps, application environment, etc.) and nothing is model, not one thing. Errors are added to a notification list that a user can look at at any time. Data merge conflicts are resolved via automatic default branching that the user can override later. Data is copy on write, and versions are retained. Login is handled with PKI.
Modal is forbidden on our platform period. If an app breaks this some how and finds a way to hack a modal event, we will treat this as a DOS attack and remove the app and ban the developer. We believe the user is the final authority, not the programmer, or the platform.
(obviously this is a bit of a pet peeve for me)
I like the cut of your jib!
I wholeheartedly agree: modal dialogs are demon spawn. A throwback to the single-threaded Mac that freezes the entire operating system even while you have a menu popped up.
A full-screen, in page overlay, in red, with a clear, black-on-white dialog stating what you were about to do and requesting confirmation.
It's obvious. It's user-centric. It doesn't affect other browser tabs. And they don't abuse it for other functions (nags, etc.).
In fact, I cannot name a single one. Open/save file dialogue? Emacs minibuffers are light years ahead of the current Windows crap.
Asking for credentials? Same thing, only grey out the controls that directly depend on credentials and use something like emacs minibuffer for entry, not a new window on top of the rest of the form.
And, while I'm still on it - don't EVER grey out anything without an explanation. I want to be able to hover over a greyed out control and see a detailed explanation immediately. Not doing this is rude.
If a UI designer cannot build a seamless handling of all the exceptional cases into a flow, he'd better move on to a simpler job (I don't know - programming, sweeping streets, packing groceries, whatever) and let the professionals do the work instead.
Also, most (if not all) frameworks implement modal dialogues in a very ugly way. For example, in Visual Studio it is a very often case when you may want to copy and paste something from some other editor window into a modal dialogue. But you cannot - it's all locked! Such things are extremely annoying and totally unacceptable.