You can specify the layout attributes in percentage (of the containing region) or in pixels. There are advantages and disadvantages to each approach.
The pixel approach works better if regions have bitmaps. However, once a region has a fixed pixel width (or height), it becomes less flexible and adaptable to browser windows of different sizes. This is the same issue as with table widths and heights specified in pixel units.
The percentage (proportion) approach makes the regions rescale themselves as the size of the browser window changes. This feature comes in handy when the content of regions are mostly text. This approach also guarantees that the entire web page can be contained within the browser window, although certain sub-regions may overflow and require its own scroll bar to scroll through the contents.