Splashpages are not always unnecessary.
As a matter of fact, if your website heavily depends on whatever technology is used, I would check up on it.
For general HTML, CSS and so on, I'd recommend the following:
CSS Validator:
http://jigsaw.w3.org/css-validator/
HTML Validator:
http://validator.w3.org/
For JavaScript, the JavaScript console (Tools -> ...) from Firefox or Mozilla browser is the best tool to debug and fix code.
Hope that helps!