ETA: Wikipedia has reminded me the feature was called UniversalXPConnect, and it was a Firefox thing and wasn't cross-browser. It still sucks that it was removed without sensible replacement.
I thought opening index.html in the browser was basically just a demo.
The other mode is with a nodejs server (it's what I'm using). This allows me to access the wiki from all devices.
I forget that File -> Save is even a thing for websites.
It's not perfect, though. Paragraphs are rendered by using two br tags, instead of p tags. Link syntax is the reverse of MediaWiki syntax; i.e. [[foo|bar]] links to "foo" in MediaWiki, but "bar" on TiddlyWiki, which trips me up constantly. There's other syntax awkwardness like sensitivity to spacing and newlines. Journals sort in alphabetical, not chronological order.
It is a common gotcha that new users will lose some of their work while they learn persistence.
With TiddlyWiki you had to essentially File -> Save As and save the HTML back over itself. There were other ways too but they were all workarounds to the issue that browsers don't allow direct access.
I did a deep dive on TiddlyWiki recently. No, it doesn't need a server; an entire site can be self-contained in one HTML file.
If you want to run it as a multiuser, web accessible wiki, it runs on top of NodeJS.
So I'm trying to understand the difference, the payoff. I understand that local web APIs are ass and you very quickly run into the need for a server.
But I'm wondering about the utility of combining the two approaches. It seems like a contradiction in terms. Here's a server to help you with your dev setup oriented around not needing a server.
I guess the main win would be cross device access? You have it online and you can edit it easily.
I'm editing my stuff on my phone in a text editor. And syncing it to my laptop with a sync app.