During development, how much testing is the right amount for a website to do?

I think it depends on what kind of software you're writing and for whom. I've worked on projects where we did all of the above with CI and on projects that did absolutely no testing. I'll probably be crucified for saying this, but if you're building a consumer web product and doing multiple releases a day, often it's acceptable for users to help you find non-critical bugs and functional testing won't be as important. You might only write functional tests for your API, for example, which you can't afford to have break or the core functionality for your product. I like unit tests to verify "business logic." These are especially important if you aren't using constraints in your database (or your data store doesn't support them) for data integrity. In addition to this, one of the biggest benefits I get from writing unit tests is that it helps me design better interfaces for my objects. Writing regression tests tends to yield a big payoff as well, since the same pieces of functionality always seem prone to error. Like everything else, you'll have to weigh the tradeoffs. I'd err on the side of testing, especially if you have more than a couple developers on the product because you'll quickly realize large time savings fixing bugs. On the other hand, if you're a sole developer working on a small consumer web project and you want to move fast, you might be OK with incurring what amounts to technical debt.

A point rarely raised up but nevertheless important is: don't test blindly ! One thing we should do is to define the test strategy at the begining. Not all kind of software need the same amount of tests (depending on how critical they are) and not all parts of the software need the same attention. I know from experience that some parts of the software required good deal of attention and they are sometimes parts not anticipated by developers. For instance: installers ! Installers are the first contact of the users with the software, it should behave correctly any time under every conditions ! On the other hand some web pages deep inside the navigation tree will not require spending too much time on testing. It may crash under unexpected condition but it would be unreasonable to spend too much effort here. In short: build up a strategy and adapt your testing effort to the context !