In the world of digital publishing, style sheet is an ambiguous term.
It originated in the print-publishing industry. A style guide (or stylebook) is a book that lists the important rules of capitalization, punctuation, some basic grammar, some spelling issues, and the syntax of citations in footnotes and endnotes. At Invisible Order, our standard style guide is The Chicago Manual of Style. Other style guides you may have used or at least heard of include the Associated Press (AP) stylebook, the guide for the Modern Language Association (MLA) — even the venerable Strunk & White probably counts.
But there are various reasons a particular publication or company may want to diverge from the rules given in a style guide, while still wanting to remain consistent. If so, they maintain a document for their "house style." To avoid confusion, in IO we call this our house style guide, but the common term from the print world is "style sheet." As the name implies, it was not supposed to be longer than a single sheet of paper. It doesn’t have to be a list of differences from the main style guide; it can also be a list of the most important rules from the main style guide. You can see ours here.
Why would it cause confusion to use the term the way the print world does? Because at Invisible Order, we do both editorial and technological work. And on the technological side, "style sheet" means CSS (i.e., "cascading style sheets"): instructions to a web browser or ebook reader for the visual presentation of text and images.
I’ve worked on teams where someone would say "style sheet," and everyone thought they knew what the term meant, but the coders thought it referred to typeface, character size, and layout, while the writers and copyeditors thought it referred to commas, semicolons, and compound adjectives.
Fortunately, it’s easy to avoid the ambiguity. On the editorial side, we use the term "style guide" to cover both the Chicago Manual and our house style. We use "CSS" to be unambiguous on the tech side. And when someone talks about a style sheet, I smile and nod and look for an opportunity to make sure I know which kind they’re talking about.