1. If you mean something like
http://mobiletest.me/ but for the 3ds then I am not sure. There probably is but not many people would truly have need of it. That said
https://www.nintendo.com/3ds/internetbrowser/specs/ is reasonably easy to work to.
If instead you can something along the lines of "can I get something like wordpress and install a theme that will allow me to do 3ds websites off the bat" then there might be something, however all I would be doing here is doing a search for you and my token search says not a lot is out there.
2. If you can not find a wordpress theme or something I imagine there will enough people around here that can give you pointers on basic HTML and CSS and reference it back to that link above. Designing for mobile devices is quite annoying though.
3. Be careful when you say HTML only -- some may assume just that where you may have many more options* and you only meant you can not use java or flash to do things. Obviously things like flash and java make so called rich applications for the web far easier, though you can also do many things with straight up PHP too. The 3ds browser only goes to HTML4, though it reckons it has some canvas stuff (HTML5) as well so you might not be completely out of luck with raw HTML. Javascript can do quite a bit as well. Trying to do chat with just HTML post is not fun at all
If you are using some of the nasty free hosts then they might well limit you to html only (no php or anything, and certainly no databases), however if you are playing on a real host then you might have more options.
If you have your own PC and just want to play on your internal network then you have things like
https://www.apachefriends.org/index.html and
http://www.wampserver.com/en/ that will run a full apache, mysql and php.
Even if you are down to HTML only and no javascript then you could always make a very nice choose your own adventure using nothing but links.
*in web design you have two main things to consider as far as running code
1) Client side. Flash and Java are two, however javascript which is unrelated to java can run on the 3ds browser (they call it ECMAScript on the nintendo.com link above for complex and boring reasons but if you know javascript then you know ECMAScript and everybody will know what you are talking about when you say javascript).
2) Server side. PHP with a mysql database is the more common setup you will encounter here, however there are many many many more options for things to do here.
As long as the client is presented with something it can render then it does not matter what happens on the server side of things.
Anyway I have probably just confused things slightly for you. Hopefully we/others can get you to where you want to be though.
