Building web page layers
The z-index is probably my favourite CSS property to work with. The z-index of a page element is its stacking order in the document. To simplify this down let’s say you have 10 div’s / blocks on your page, each the same size. Div’s are block level elements and each of them would struggle for their nearest space and push the next one below it leading to a line of div’s down the page. By placing a position like absolute on the div’s and setting z-indexes on them you can place them on top or below each other in whatever order you want. What you can consider the default document z-index is 0. Your actual page will be at this z-index, and you can develop your elements to sit above or below it, or you can technically set the default document z-index to a different value by using a container. There is a number of ways to position z-index elements, by using a margin etc. and by setting a relative position on the containing element.
Absolute positioning can cause issues in some browsers and setting z-index properties on all your absolutely positioned elements can help stop these bugs. When you’re actually positioning your elements take care about things like menus and fixed elements. If you have a document with a complex stacking order make sure to test how fixed elements stack against page elements as the window scrolls.
CSS3 and the z-index
A lot of CSS3 effects are based on the z-index and stacking. You may find across the web complex and very beautiful buttons with animated hover effects, borders and shadows. A lot of these will be built using :before or :after pseudo elements and stacking using z-indexes. You can actually place div’s within div’s and animate the layers of the stack in different ways. You can also change the z-index on hover and other events and create some interesting effects. There is no end to actually what you can do and the only way to experience the power of the z-index is to experiment. Think of something crazy and complex that requires different layers and try to build it.