One of the key characteristics of an "expression" is its concrete nature. If something is not concrete, it isn't protected by copyright.
From the United States Copyright Office, Copyright Basics:
Copyright protects "original works of authorship" that are fixed in a tangible form of expression. [Emphasis mine.]
For instance, if somebody makes a speech and no recording of it is made, there is no concrete representation and thus no protection. If a recording is made, then it is protected. Since there was no such thing as a non-concrete expression when these laws were made, many definitions don't even talk about it.
The problem is, software does not have to be concrete. At all. Not even close.
With all the various browsers, all the various personalization options, and all the various times that people visit, it is possible that no two people visiting a modern dynamic website will see the same combinations of pixels, even if they were sent exactly the same expression (HTML code), which may never happen. With many browsers, a simple keypress like "CTRL-+" or "CTRL-" can generate a different looking page then the one you are looking at now without receiving a new communication at all! Yet clearly, there is a pattern of similarity there; clearly there is something concrete there that should be protected. Even with a static web page, all browsers receive the same raw HTML source, yet the appearance of the site may change drastically from browser to browser. So we might guess that the set of all possible renderings of a site is also protected under copyright law. Unfortunately, for even a single page, that is a large set, and there are a lot of overlaps. For instance, imagine two pages that are identical, except for an important image which is different. Both pages can be freely rendered without images by a text-only browser, thus the representation for those pages would overlap, despite the fact the two pages are distinct.
We need a cleaner way of thinking about these amorphous expressions. It should probably match up to our intuition of when sites are 'stealing' from each other, because our intuitions are quite clear and pretty much everybody agrees that when it comes to online theft of design, layout, or even content, they know it when they see it.