I'll recommend to test with the greatest number of web-browser, but for a personal-development, it just impossible to do.
So i think, the easiest way is to follow the "web-norm" : if your page is valid (thank's to link like
http://validator.w3.org/), it will be ok on 99 % of modern browser.
For people still using out-dated browser, have a look to alternative. It's not because XP got ie6 that you must not use something else.
For web-developer, you can use Watir (
http://watir.com/) test on your project, and you can also check a web-browser with it :
http://watir.com/platforms/To sum up (a little bit), It's depend of what you use to create your page.
If you use recent technologies (like html5 / css3), you need to check only recent web-browser.
Else, if you use more classic technologies (xhtml/ html4 / css), i think, you just have to validate against (at least) :
* IE 8 (because Xp can't have ie9 -and because ie9 is much better than the 8th)
* Firefox 3 (because more recent version won't be a problem if this one is ok)
* Safari 5 (because chrome move too fast and it use the same engine for html)
If it works fine on safari & firefox, opera & konqueror won't give you trouble.
Other web-browser mostly uses one of previous engine or they are practically never use.
About ie : ietester can be very useful, especially on XP.
Edit :
http://browsershots.org/Here you can check the display of your page on multiple webbrowser. It's not perfect, but it's the easiest & fastest way i found to validate css/display.
Edit² :
About difference between web-browser :
http://en.wikipedia.org/wiki/Comparison ... rd_HTML%29http://en.wikipedia.org/wiki/Comparison ... _Sheets%29For only "big" differences.