Website builders have different browsers to test their web pages. The most common browsers are Internet Explorer and Mozilla Firefox. In late Aug, 2008 Google added their browser Chrome to the fray and no doubt it will make it's presence felt. Lesser known browsers include Netscape, Opera, Konqueror, Safari, Flock, SeaMonkey and Epiphany. (there are more too!)
Testing involves using the latest versions and earlier versions of browsers. If Your Site works across all browsers, or at least across the popular ones, you can eliminate browser problems as a cause of low visit lengths on certain pages. Explorer and Firefox - the two most popular browsers - get updated automatically, so if your site works in those you should be right but it doesn't hurt to test it in others.
What comes back is worth a look at: a page full of thumbnails of Your Site which automatically enlarge when you mouse over. It's a cool exercise when you submit and as mentioned it's free.
Obviously, the service has its limitations but at least you wont be trying to chase up old versions of browsers yourself and download them. If you do want to find older versions of browsers, a good start is either one of the following two resources:
If you have the main two browsers Internet Explorer and Mozilla installed you wont go far wrong and will be able to extensively test Your Site yourself. As a minimum anyway, those two main browsers, and Google's Chrome (prediction) should be on your computer as the majority of your visitors will use one or the other. However, I wouldn't fret too much if you don't install some of the obscure browsers though it might make for an interesting exercise if you do try them out.
W3C is the official organization overseeing issues regarding code to build websites. W3C have a validation service which you can use to test the code in your pages. In their own words the "validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc"
From my experience the validator is very strict and there is probably more you can get away with in writing code. This is the impression I have of the W3C overall. They are definitely sticklers for protocol which is their job after all! Nevertheless, using the validator will tell you what code is not officially acceptable and in the process may highlight mistakes you might have made.
Script and other errors or bugs can lead to very slow download times and there seems to be a general consensus that you should try to put Javascript and CSS Style code in separate files rather than inline on pages you build. Inline means the browser must read the script each time the page loads. And if you have any bugs or errors in either inline or external files your pages will take ages. Code errors can be easily overlooked and browsers typically ignore codes that don't make sense to them. If you forget to cross a t or dot an i the computer won't just do it for you it will give you an error instead. Computers are very dumb really, because they take you literally! So check codes and also links in your pages to see they're doing what they are designed for.
Two ways I do this. Either manually check links or go to cpanel logfile of errors which will give a record of requests on the server that produced an error in relation to my website.