A big part of the focus of this years conference was Mobile devices and the impact they will have on the web. Ajax, CSS, Accessibility and Social networks also featured highly. Many of the slideshows are available on SlideShare.
After the failed upgrade attempt a fortnight ago I spent a lot of time looking in the wrong direction. The main problem was that users were unable to log in. So I thought it must have been session related. I tried many experiments with cookies, cache and sessions locally but couldn't workout what was going wrong. Everything seemed to be working correctly apart from the failed logins.
z-index changes the stacking order or layering of positioned elements. If no element is positioned then the stacking order gets higher, closer to the front, for each descendant.
Each positioned element that has been assigned a z-index other then auto, creates a stacking context, which may be easier to explain in another context lets use a sandpit. The sandpit is positioned absolutely so the children don't move it an make a bigger mess and has a z-index of 1.
CSS gives web designers the power to style many pages simply and to change the look of a whole site in one place. If you look at a site as a whole you can simplify your style sheets and make them smaller and easier to maintain.
Let's look at the basic structure of a rule to make the rest of the explanation easier to understand.
There are different types of selectors which I am not going into here except to say that you can have multiple selectors separated by a comma.