Actually html is largely a "spinoff" of SGML.

SGML = "Standard General Markup Language" if I remember it right.

SGML provided for "tags" that would indicate the usage of text and of things within it, so that format could be applied to the tag and everything with that tag would look the same, and so that searching "by tags" could make it easier to find and/or organize what's relevant.

Being an attempt to be logical, it naturally faded away, although it still remains in use in a few places, and the Standards Committee still tries ...

HTML, or hypertext markup language, continues to use the tags to indicate "classes" of "objects" but the emphasis has shifted away from classification by use in the direction of classification by intended appearance. It remains an abominable "language" for controlling "format" and "appearance" but people keep trying to push it in that direction. The "invention" of nonstandard tags "to provide cuteness" by browser makers, and their varying application, is one of the reasons that some browsers can see Scrolling Lines while others can see Blinks, a few can see either, and some can see none of the above.

Syle Sheets were an integral part of SGML, but the CSS sheets used on the web are another extension more for display, format, and action than for logical structure.

An XML "extension" of the HTML bastard child of SGML extends the kinds of tags even further toward permitting one to define things that have no logical significance and contribute nothing to "information" but provide movement and annoyance (and lots of malware).

One supposes it will all evolve into something helpful someday ...

John