About Web Browser Compatibility
Nowadays, it is still assumed that most internet users are browsing the internet using Internet Explorer, and whilst this is true to a certain extent, a growing number of users are starting to use other browsers such as Firefox. The split is currently, according to a June 2008 survey, that roughly 27% of people browsing the net are using IE7 and 26.5% IE6. This means that a total of about 54% of users are using Internet Explorer 6 and 7, 41% are using Firefox and the remainder are using other browsers.
As you may know, IE6 handles websites differently to the way IE7 handles websites. Firefox and the other browsers* will also handle websites in "their own way". As a result of that, your website should, as an absolute minimum, be tested on all of the primary platforms, and, if possible, on the minority browsers too.
An example on how tricky it can be to make your websites look the same in multiple browsers, is the narrow text on the opaque background to the right. W3C specifies that a rigidly sized block box should allow oversize content to protrude or overflow beyond the edges of the sized box, but IE does not obide by those specifications. You can read more about it here. As a result of that, we had to make a few "tweaks" to the stylesheet to make it behave how we wanted it to.
As it is, just having the opacity applied makes the CSS fail the validation, but I think the clients, in this case, are ok with that.
At Purple Pixels we thoroughly test all the websites we design and develop. We endeavour to make our designs HTML/ XHTML and CSS, validate in WC3 according to assigned doctype, to the extent this is possible. If, for some reason, we are unable to make valid coding on a page, we will inform the client, and we will make sure the page does not claim to be valid according to the WC3 validation rules.
*The other 5% of web browsers includes opera, which is now the web browser used by the Nintendo Wii and some mobiles, and also Safari, used on Apple Mac's, iPods and iPhones.