It exists. It's called Content Management Software or sometimes Typesetting software. And one of the big players actually started over by Mongoose in Swindon, UK. It's now called Arbortext and owned by PTC. There are others.
Nobody in the RPG sphere uses CMS because it's hard to teach creatives new tricks or to get them to comply with standards. Teaching them a new tools where they enter the words and then the visual aspects are controlled by someone else via style sheets and similar goes against their grain. Or, on a not so pessimistic reason it's because the CMS apps target technical writing as their market and not creative writing. Because large companies that do technical products are willing to spend the large licensing costs and setup costs involved, and RPG companies are not.
But a CMS system can easily take the same content and format it for a printed book, a web page, ebook format or multiple VTT formats. And a change in one place updates all the downstream formats.
But not cheaply. When I was involved a decade ago with Arbortext, a typical setup started at about $250,000 USD with annual license costs of $10k/user. No idea what it is now, but you can see its substantial.
HTML does not require an internet connection. It is simple a digital format designed for web browser use. Web browsers can open local or web served html files (pages).
Yea? But then again so do all the eBook formats like ePub and with none of the fixed format problems of a PDF.
I get that you like PDFs, probably because they are easy to come buy and its what you know. But they are a horrible digital format for doing anything but preserving a printed page format.