Why is a pseudo-class so called?

Question

a:hover

Why is it called a \"pseudo-class\"?

Are there any similarities with the concept of \"class\"?

Answer 1

From the w3c CSS2 selector spec:

CSS introduces the concepts of pseudo-elements and pseudo-classes to permit formatting based on information that lies outside the document tree.

• Pseudo-elements create abstractions about the document tree beyond those specified by the document language. For instance, document languages do not offer mechanisms to access the first letter or first line of an element's content. CSS pseudo-elements allow style sheet designers to refer to this otherwise inaccessible information. Pseudo-elements may also provide style sheet designers a way to assign style to content that does not exist in the source document (e.g., the :before and :after pseudo-elements give access to generated content).

• Pseudo-classes classify elements on characteristics other than their name, attributes or content; in principle characteristics that cannot be deduced from the document tree. Pseudo-classes may be dynamic, in the sense that an element may acquire or lose a pseudo-class while a user interacts with the document. The exceptions are ':first-child', which can be deduced from the document tree, and ':lang()', which can be deduced from the document tree in some cases.

So basically, a pseudo-class is something you can attach a style to, but you never print it out yourself in the HTML. Also, a pseudo-clas can be aquired and lost depending on user interaction with the UI.