I used TiddlyWiki a lot to manage my D&D 3.5 campaign back in the day. As I recall, it originally was a true stand-alone HTML document capable of overwriting itself, but once browsers dropped support for this capability, users had to begin using various workarounds, and this remains the status quo today.

TiddlySaver.jar was one such workaround. A check in the Wayback Machine suggests that it was originally required only for Safari, Opera, and Chrome; IE and Firefox needed no such plugin. Nowadays, there are several workarounds, and setting up one is a mandatory installation step: standalone applications, browser extensions, servers, etc. Some are clunky (e.g. you have to keep your wiki in your Downloads directory or the browser can't write to it), and either way, TiddlyWiki is no longer truly a single stand-alone HTML file, at least not for writing purposes. It's still a very versatile tool, though.