A style sheet language, also known as style language, is a programming language used to define the presentation layer's UI design rules of a structured document. We can think of it as a skin or a theme that we can apply to a logical item (the structured document) to change its appearance. For example, we can make it look blue, red, or yellow; we can make the characters bigger or smaller and thinner or wider; we can change the text spacing, alignment, and flow; and so on.

Using dedicated style sheet languages gives developers the chance to separate the presentation layer's code and structure (respectively JavaScript and HTML) from the UI design rules, thus enforcing the separation of concerns (SoC) principle within the presentation layer itself.

When it comes to web pages, web applications, and anything else that mostly uses HTML, XHTML, XML, and other markup language-based documents, the most important style sheet language undoubtedly is CSS.