It’s time to start writing your CSS

At the begining, give each div a border. For example, div {border: 1px dotted gray; padding: .5em} This will help you see where they begin and end, and also whether or not you have any nesting going on.

Write your CSS for element selectors first (<html>, <body>, <p>, <h1>, <h2>, <ul>, <li>, etc.)

Use contextual or descendant selectors as much as possible. This will keep your markup much cleaner. For example, #subnav li {border: 1px solid black; padding: .5em; display: inline} will only affect list items that occur within your subnav div.

Test in as many browsers as you can and get your friends to test it in their browsers.