What exactly is hypertext?

Hypertext is common text with other aggregated information. According to Wikipedia :

  

Hypertext is the term that refers to a text to which are added other sets of information in the form of blocks of text, words, images or sounds, accessed through specific references in the digital media called hyperlinks, or simply links.

The idea of hypertext is not restricted to the digital medium, however, a simple book with footnotes [1] can be considered hypertext. HTML is the most common language for expressing hypertext today, but there are others, I would say for example Markdown (the same language used in StackOverflow's questions and answers) and BBCode (much used in discussion forums) are also hypertext. Extending the concept to other media - such as images, videos, etc. - we come to what is called hypermedia > (think of YouTube videos with annotations, multi-language subtitles, or external links within the video itself).

About CSS (and JavaScript), I'd say no, it's not a form of hypertext. Nobody (no human reader) consumes CSS for its content itself, it is always modifying the appearance of another main content. Similarly, a programming language was made to be run, unread, and other text-based formats (such as XML or JSON) are intended to store structured data for use by other programs , not by the end user.

The fact that PHP is currently a "recursive acronym" for "PHP: Hypertext Preprocessor" does not mean that it is restricted to preprocessing hypertext - it is a general purpose programming language capable of executing even outside the context of a webserver and handle data of an arbitrary nature (including binary data). But as its "strong point" is to generate hypertext, people soon realized that it was also capable of generating other text-based file types, hence its use for those purposes.

[1]: How are you?

According to w3.org :

  

Hypertext is a text that is not limited to always being linear.

     

Hypertext is any text that contains links for other texts. The term was coined by Ted Nelson in 1965.

Wikipedia in English Hypertext :

  

Hypertext is any text displayed on a computer screen or any other electronic device that contains references ( Hyperlinks ) to another text that can be accessed immediately by the reader (...)

In other words, hypertext is any text with external references, whether they are pointing to a local directory or the web.

Now, if CSS is hypertext, well, I do not think so, but I'll let someone more experienced answer. About PHP, I think it serves to generate hypertext (among other things), but I also do not believe that by itself can be considered hypertext.

Correct me if I'm wrong.