HTML Studio was written back when HTML editors were still relevant. Before Python, and web frameworks, and Web 2.0 and Ajax. At least, that's my (probably flawed) perspective.
In those days, there was even something that you could perhaps call a scene, a subculture of some sort, within the German webdesign community (we are talking ~2003/2004 here). There were a handful of freeware HTML code editors, mostly done by amateurs (in the good sense), but all dead by now.
The one with the biggest fanbase, the most widespread use, the editor to rule them all, was, without a doubt, one called Uli Meybohm's HTML editor (sometimes also referred to as phase5). It certainly was one of the first, and most other tools (including this one) were at one point or the other accused of copying it. Of course, phase5 in turn adopted many things from Nick Bradbury's Homesite, which was later sold to Macromedia (which was later sold to Adobe). And Nick is now responsible for the excellent Feed Demon.
Anyway, I wrote an HTML editor myself, it was even moderatly successful, but is now old, outdated, potentially broken on new Windows versions, and generally useless if you want to do more than just HTML. And even if you don't, it shouldn't be to hard to find better alternatives (seriously, go look).
Still, I'm putting both the first and the last version up here for you download enjoyment. In the unlikely case that you are interested, note that the program is completely in German.
There are also a number of plugins available - or at least, there were. Some of them are probably still around, just search the web, I won't list them here. In addition to it's own, HTML Studio supports the plugin interfaces of Phase5, Weaverslave, HTML Pad, and EPS, so that's were you should go look.
I'd like to give a shoutout to the people that supported the project via donations. Namely, in chronological order, Andreas Losert, Holme Computer Systeme, Computerline troxler, Reher Elisangela, Thaler Anton, Jürgen Keller, Paul Mundy, Frank Heiko Bauman, and the one guy who wanted to stay anonymous. Thank you.