• The advantages of this are obvious - but once again Netscape has very limited support of CSS layers - and top it off: the limited support it offers is quite often executed with failures.

• So the real challenges when working with layers is to make them work on Netscape browsers as well.

• To create a layer all you need to do is assign the position of the attribute to your style.

• The position can be either relative or absolute.

• The position itself is defined with left and the top properties.

• Finally, which layer is on top is defined with the help of z-index attribute.

You can either position your layer calculated from the position where the layer itself is inserted (relative) or calculated from the upper left corner(absolute).

If you define the position to absolute it will be calculated from the upper left corner of the page - unless the layer is defined inside another layer, in which case it will be calculated from the upper left corner of the parent layer.

If you define the position to be relative it will relative to the position of the tag that carries the style. That is, if you add a relatively positioned layer in the middle of the page, then the position will be calculated from that exact spot in the middle of your page where it was added previously.

• While the position property indicate the out spring of our coordinate system, the left and top properties defines the exact position of our layer.

• You can enter both positive and negative values for these properties - thus it is possible to place contents higher up and further to the left on the page than the logical position in the HTML code where the layer itself is defined.

• In other words: at the bottom of your HTML codes you can enter the code for a layer that is positioned at the top of the resulting page.

• Both top and left properties can be dynamically changed with JavaScript.

This means that it is possible to move thing around on the screen even after the page has finished loading.

In fact this technique can be used to create entire games. Other uses might be menus that pop out when a mouse-over is detected on a link. The possibilities are endless - but in order to keep things simple, we will not dig into details about these dynamic HTML effects here.

• Picture a game of 52 cards. If the ace of spade was at the bottom we'd say it had z-index:1;. If the queen of heart was at the top we'd say she had z-index:52;.

• Try looking at the code examples at the top of this page again, and see how we used the z-index to put LAYER 1 on top in the first example, while we had LAYER 2 on top in the second example.

• Very interesting possibility arise from the fact that the z-index can be dynamically changed with JavaScript.

• It is possible to create several "pages" on top of each other - all on the same page. When the user clicks a link it will simply move the layer with the desired info on top rather than load a new page. The technique to create effects like that goes beyond the scope of pure CSS however, so for now we will just refer to DHTML (Dynamic HTML - a mix between JavaScript and CSS) for further explorations into that area.

• A final property is the visibility property that will allow you to create invisible layer.

• Why would anyone want to create an invisible layer? Well, imagine the possibility it gives for adding pop-up menus and other cool effects on your pages.

• With dynamic HTML it is possible to change the visibility of a layer according to certain events. The most common use of this is to create menus that pop out (like the sub menus in the START menu on Windows). The trick behind these menus is to create all submenus as invisible layers. Then, when a mouse-over is detected on a link the according layer becomes visible. (Sounds pretty easy - actually is pretty easy - except when tried on Netscape browsers that seem to have only idea of the logic behind CSS layers).

• Valid values for the visibility property are:hidden and visible.

This example shows how to create an invisible layers:

• It's obvious that layers offer certain possibilities for precise positioning of static element on your pages.

• In reality layers are often used in more dynamic way:

  1. Flying elements/banners on the page
  2. Games where you move an object around
  3. Menus that pop out when triggered
  4. Menus that become visible when triggered

• While all of these effects might seem pretty useful - the fact is that the web is filled with dynamic effects that are much more cool than the average visitor really likes.

• The more you can create a unique interface for your site the more you force the visitor to forget about what she is used to. Do not underestimate the power of sticking to the elements that the average visitor is accustomed to.

• What is cool about creating an effect that makes 90% of all web designers clap their hands while leaving 90% of non-web designers confused or disappointed?

• In any case, judge for yourself if certain effect is really needed - and if so: do not hesitate to use it.