I've always found it ironic that this library calls itself "Semantic UI" but doesn't follow the practice of semantic HTML/classes[0]. W3C suggests[1] that classes should be used for semantic roles (e.g. "warning", "news", "footer"), rather than for display ("left", "angle", "small" -- examples taken from Semantic UI's docs). So instead of giving a button the class of "button" it would be better to give it a class suc…
I always found that "semantic" approach very misleading. In computer languages "semantics" is what the computer does with your input. This is the semantics of C, for example. Markup languages are no different. Until you have a clear specification what computer does with the markup, you have no semantics. I.e. , , and tags have perfect semantics, while things like or have none or very vague wishy-washy one.
On the web, though, "semantics" is applied to the problem domain; i.e. "what application am I trying to build with this code", i.e. "what is the meaning of this word with respect to the final user-facing application". Thus, for HTML you get words for the things that are typically build with HTML: web pages. Then you get tags for articles, images, quotes, headers and footers, side bars, etc. These describe the types of content in the page, not how they look.
The "semantic web" is a generalization of this "domain knowledge" idea, providing tools to define your own vocabularies and build applications with them. This approach amounts to creating your own programming language (a new one for each application) in a way that best suit the problem at hand, rather than using a general purpose programming language.