Let me try(?) to break down and answer as much as I can.

To my knowledge (based on my own personal testing) with Lioden HTML on forums, I can safely say that forum HTML is different from webpage HTML (and in extension den HTML) mainly because CSS stylesheets do not work here in the forums. In other words, you cannot use CSS styling from a different part of the internet on your forum posts like your den.
Edit: Instead on forum posts, you use inline CSS (where you apply a unique CSS style to a single HTML element)

You don't need to define doctype in forum HTML, nor do you need to explicitly add HTML tags. For example, I'm linking this to your den using just an 'a' tag.

As for how people make boxes on the forum.. there are two ways you can use. You can either use a table tag or a span tag. Each has its own pros and cons. Personally, I use a mix or either/or depending on what kind of design I want to experiment with.

There are many good resources here on Lioden that teach you what various HTML tags do. In fact, the three stickied threads here in HTML help are good resources you can use to figure out what each tag does. After you figure out what each HTML tag does, the next step would be to see how to combine various HTML tags.

Hopefully this helps out, but feel free to DM me for any help.

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