We have already defined what is going to appear on each screen by using wireframes. However, we also need to consider issues such as accessibility, browser compatibility, screen resolution, and performance of the system as a whole.
Browse rs are always changing; what was standard two years ago is different to now. However, what's important here is what browsers your client will require you to support now, and in the future. Simply accommodating the most popular browsers now will mean that when the newest browser becomes the most popular, there's a good chance rework will be required, e.g. when IE7 first came out, the adoption rate was slow, but soon it became far more popular. Creating a solution that catered for IE6 only would mean that in a short space of time rework would be inevitable.
As there is no true standard set of browsers with which to be compatible, you'll need to firstly confirm any specific requirements set by the client...