Test layer
Development stack
Test twice, run once. Every line of code has it's purpose, and every line of code needs to be exercised twice: once in the affirmative and once in the negative. Good software engineering practices require this type of unit testing because once a system is deployed you don't want to be making changes.
But a good testing regime requires much more than unit tests. Consider these other important elements: system testing to make sure the pieces interact correctly; stress testing to understand how the system fails under heavy loads; link validity testing to make sure that the Internet flux hasn't changed since you last visited your out-of-site hyperlinks; and standards compliance testing to make sure your CSS, HTML, and JavaScript pass all validation checks.