Because the amount of code involved in many site improvement tasks is often considerable, software engineering considerations should play a role in your design process. Large sites with numerous menus and a large number of pages are much easier to manage when your code is reasonably organized. They’re even easier to manage when the code is written, maintained, and distributed by someone else!
Suppose you wish to use a hierarchical menu system on your site. The amount of time it will take to change the entries of these menus to reflect a new site organization will largely depend on how the menus were implemented. Making such changes can be a very laborious task if all your code resides inline in each individual page. It is for this reason that DHTML coders for large, professional sites and libraries spend almost as much time thinking about ease of use for the programmer as they do thinking about utility for the user.