Easier to maintain
Just as project management software (mentioned in Chromatic Sites article "12 Steps to Creating a Professional Web Design") simplifies collaboration within web-based projects which have several people onboard, so too does CSS simplify projects with multiple contributors. Rather than having to sift through page after page of junk markup found in table-based layouts, designers only need to open and edit the external CSS style sheet. In addition, as long as members of a team use descriptive selector tags in the style sheets, then it will make editing much easier for additional team members to go in and edit the file without needing to be briefed and educated about the style sheet's contents and structure.
To better explain descriptive selector naming, here is an example of a few elements from a style sheet:
#mainContentLeftColumn { property: value; }
#mainContentMiddleColumn { property: value; }
#mainContentMiddleColumnLeftColumn { property: value; }
As you can see, the deeper into the web site's structure you go, the more detail the selector names have. With these descriptive titles, a team member who knows very little about the layout of a website or organization of a style sheet should easily be able to find the exact selector(s) in the style sheet and edit them accordingly without having to make guesses about the meanings of different selectors. Not only will this save your team time in the long run (as previously mentioned, each team member won't need to be briefed on the structure of the style sheet), but it will also save time for an individual working on a project by themselves.
With table-based layouts, there is no advanced selector labeling available. Instead, tags such as table, font, tr, and td plague the content documents. Since these elements cannot be labeled, this can make table-based designs a nightmare for teams and individuals to manage.