As fledgling Moodle themers or website designers, you will need to understand the importance of browser compatibility, which is the need to make sure that your themes or designs all look similar in the many different browsers available in the market.
In most cases, the variation from one browser to another will be minor and probably not even noticeable for the average visitor. But, in some cases, especially when using CSS, you might find that the differences will prevent users from using your Moodle site. The best way to make sure that your site is usable to most of the Internet audience is to test and then test some more.
One of the first things that you will need to do is to find out which browsers are the most commonly used by your visitors. If the Moodle site is an internal educational or corporate site, this will be relatively easy, as your IT department will probably allow only one or two different web browsers to be installed...