From CSS Discuss
The often-posed question is "which browsers should I use to develop and test?". Everyone will (indeed, must) make a different choice, according to their preferences and circumstances, so this document suggests some generic advice. See also Browser Testing for the mechanics of gaining access to multiple browsers.
It's worth developing (and testing) on the most standards-compliant browsers whether or not they have a significant market share since they represent the norm or ideal (well, almost) towards which all browsers are moving (some faster than others). If you aspire to cross-browser robustness, it's not an efficient strategy to base your initial development on the most widely used browser (viz. IE6).
Standards-compliant browsers include Opera and Gecko-based browsers (principally Firefox) and I'm assured that, on the Mac, Safari is also standards-compliant. Although the standards compliance of IE7 is a great improvement over that of IE6, it still falls a long way short of the other browsers just mentioned (not just in terms of bugs but also in CSS features not yet supported and in proprietary features that it does support but which it is not advisable to rely on).
For convenience, you will probably want to focus on only one browser during development, since you're trying to combine site robustness with speed and efficiency of development. I don't think it matters which of the above you choose. If a site works in one of these browsers, it is likely to render very similarly in the others. That's the value of standards! At suitable points during development, however, check with your other target browsers (as in Step 2) to see how your structural approach is progressing and what the issues are before completely committing yourself to any one method. For a publicly available site, it's not efficient to completely develop a site using an advanced browser and only then worry about the browser that 70% of the world actually uses.
STEP 2 The second step is to detune or add workrounds for the other browsers you wish to support (e.g. the lowest version of IE you wish to support). This will depend, in part, on what you can learn from your server log files (for an existing or similar site). This step is very variable because it depends on your attitude towards support for older/poorer browsers and whether you wish to offer graceful degradation for their users. If you don't have relevant log files, then Browser Stats may help you.
STEP 3 The third step is to re-test on all target browsers (again, depending on what your logs tell you) and to extend testing to other platforms/OSs (and perhaps other media, such as handheld devices).
This framework should give you a sound starting point but it's not rigid and must be adapted according to your circumstances. If your site's pages are going to be based on a standard template, you will want to go through all three of the above steps, using the template, before replicating the actual pages. See also the excellent and comprehensive Browser News /res_testing.htm Testing Sites on Charles Upsdell's Browser News for further advice.