We wanted to follow universally agreed standards of web design as much as possible, to avoid tying our web pages to specific browsers, make them accessible to people with disabilities or over low-bandwidth connections, and make them compatible with future developments in HTML coding.
We did not want to be limited to a single type of web design software, such as FrontPage. A collection of pages had to be maintained by several different tools of varying degrees of sophistication, down to simple text editors. External style sheets are easily maintained in with simple tools, and web pages require minimal changes to be linked to style sheets.
We wanted to apply a consistent appearance across a collection of different pages. While using a template page as the starting point for the design of each page would have done this, we were not sure what would be best for the final design, and modifying a template would not have propagated changes to the individual pages; changes to the style sheet, however, would immediately affect all the pages using it.
We needed to share the work of page creation between several different people, but needed a convenient common reference for our department's web style, which someone could nevertheless modify when necessary. A global external style sheet could work as the department-wide stylistic recommendation, and local style sheets and page-specific styles could override it in specific cases.