Dynamic HTML: The Definitive Reference

Chapter 4. Adding Dynamic Positioning to Documents

Cascading style sheets, as described in Chapter 3, are primarily concerned with how content looks on the screen (or how it looks on a page printed from the screen). An extension to CSS, called CSS-Positioning (or CSS-P), is primarily concerned with where content appears on the page. CSS-P is blended with regular CSS in the CSS2 specification, but because the Version 4 browsers were designed while positioning was a separate standards effort, I use the CSS-P term frequently.

The CSS-P recommendation from the W3C focuses on the HTML code that authors put into documents to govern the position of elements on the page when the browser-controlled flow of content just isn’t good enough. To accomplish element positioning, a browser must be able to treat positionable elements as layers^[2] that can be dropped anywhere on the page, even overlapping other fixed or positionable elements—something that normal HTML rendering scrupulously avoids.

The notion of layering adds a third dimension to a page, even if a video monitor (or a printed page) is undoubtedly a two-dimensional realm. That third dimension—the layering of elements—is of concern to you as the author of positionable content, but is probably of no concern to the page’s human viewer.

While the CSS-P recommendation offers a cross-platform way to lay out positionable elements, the browsers have extended the idea by turning positionable elements into scriptable objects whose properties ...