here are my recommendations for every web developer - it's mainly for testing of templates and parsed frontend results.
- Use FireFox - also on Mac. All other modern browser has built-in developer tools too but the best are the following addons - only in combination with Firefox.
- HTML Validator addon - that is the best thing to check validity on the fly. Use HTML Tidy algorythm.
- And now the beloved Firebug - you can check any element on the web site, trace GET/POST requests.
- Then comes the best Web Developer extension addon - you get all information about the current page - cookies, images, headers, CSS and so much more...
- Take User Agent Switcher addon by that author too.
- And if you are an expert - check Google's Page Speed addon for Firebug.