Developing a web page was and still a fun task, but one of the main problems that may face a web developer or as we said these days “Web UI Developer” is the variety of the browsers in the first place, as web developer must learn how to display his page on seven different browsers and each browser has something to force the web developer to work around.
I was reviewing and analyzing the browsers statistics on W3Schools (http://www.w3schools.com/browsers/browsers_stats.asp) where you will find that the main browsers that we MUST support are Internet Explorer (39%) and Firefox (45%) and if you want to support an extra browser so you have to support Chrome (7%)
Microsoft has announced that they will support IE6 until 2014, what that mean to web developers? In my opinion this is not affecting them by any mean, as the main factor that forcing them to make their web pages IE6 friendly is the percentage of visitors (13%) and not anything else! Also IE8 (10%) must be supported beside IE7 (15%). As you can see the differences between each other is very small and you cannot select one over the other.
On the other hand, building a page that is supporting Firefox must include FF 3.0 (24%) and FF 3.5 (21%), and no need to support FF 2.0 (< 2%) any more.
Regarding the screen resolution, the statistics shows that most (93%) of the internet users are using 1024x786 or higher resolution.
But please take care, all the above percentages may be changed dramatically if your page will be a part of an intranet site for example, as the numbers may be changed to 100% for IE6 on 800x600. So it is depending on collecting the right information from the customer before starting on developing your pages, as you will save too much time and serve your customer better.
Finally, it is not a matter of colors and shapes that must be exactly the same on all the browsers, it is all about talking to your visitor the best way you can, as this is your main role as a web developer, and as you started your site by reading numbers representing the percentage of each browser usage, develop your page the way that will make the maximum number of visitors enjoys your site.