Ok, so I have finally figured out how CSS works.
As a test, I bashed the main Great Castle Wilson site to use CSS for all structure and formatting instead of the old code, which was a bunch of nested tables and embedded text attributes. The new version of the site is quite similar to the old one, with a small number of “upgrades” to the appearance, such as rollovers in the nav bar.
What they say about how using CSS reduces the size of the files is true, even for small pages. The old index.html file was 4,068 bytes. The new index.html+style.css files together are only 3,121 bytes.
Anyway, for reference, the old index.html file can be found here.