It will be important to get some guidance on what the requirements will be from you, also. I mean CSS (Stylesheets) is a set standard and you can find a lot of information through the w3.org site. Also, they should have a validation tool there that would allow you to verify that the page conforms to standards set for the version of CSS used in the page.
Finally, you need to know what styles will be applied to what content or objects on the page. This will allow you to say whether or not the object is actually appearing correctly on-screen, across different browser platforms, per the requirements document.
That still remains pretty broad, but if you go back and forth with the person in your organization steering this iniative then they should be able to let you know the scope of your responsibilities.
I think as well you need to be familiar with the cross browser issues involving CSS. IE6 and IE7 render box properties differently. As well, Firefox adds padding to box properties. But I'm sure if you do a search you can come up with a nice list of things to look out for.
As far as standards, that's an issue that your company may or may not want to follow.
Oh yeah, most definitely, you will also find that there are some really cool css tricks you can do with CSS on IE, such as gradients, that you can't do on the other browsers, so cross-browser stuff is HUGE!