Both the specifications and the files or programs made according to them have to be maintained. There is hardly any data or service that never needs updating, moving or converting. Sometimes there is a program that can make the changes, but there always comes a time when a major change is needed and a human has to be involved. At that point it is good if the thing is of manageable size and has a fairly clear structure. After all, it may have been a long time ago since human eyes last saw it, and the original author has probably long since changed to a better job. It is probably unavoidable that, for example, an XSL style sheet is harder to change than a CSS one, but even so it is good that the designers of XSL spent some time on the right choice of keywords, provided a syntax for comments and allowed the code to be indented.
Of course, there is no help for authors (or programs!) that generate unreadable code. If you know the HTML format and you have looked at the source of some pages on the Web, you have probably wondered occasionally what monster was able to mangle the HTML code behind a simple Web page or an e-mail message in such a way that it is hard to believe that HTML was ever called a "structured language." Given the opportunity, some people will make a mess of anything.
Taking CSS as an example, CSS has many features that are there for the benefit of the people who maintain style sheets: the "@import" allows splitting large style sheets in logical units, the grouping syntax for selectors and rules allows keeping things in one place that the author expects to be changed together, shorthand properties provide a convenient (and short) way to set in a single rule several properties that usually occur together.
On the other hand, CSS stops short of even more powerful features that programmers use in their programming languages: macros, variables, symbolic constants, conditionals, expressions over variables, etc. That is because these things give power-users a lot of rope, but less experienced users will unwittingly hang themselves; or, more likely, be so scared that they won't even touch CSS. It's a balance. And for CSS the balance is different than for some other things.
But in fact, even with CSS, the power-user isn't left without means completely. There is also the DOM, that provides access to CSS through each person's favorite programming language. That's modularity.