as someone who made a tutorial like. a couple years ago for this exact thing, i can answer. some of these questions ish?

basically converting the ubbc is.... generally easy. but the big issue is when you start to get into the plugin workarounds (and into some of the other more advanced coding, like radio buttons or check boxes or onclick or anything with javascript).

the reason for this is because more or less. all of the css styling needs to be taken out and converted into the newclass coding, which is. possible but p annoying. nospaces would need to be something you keep in mind (since it would make anything with an automatic linebreak p moot). and then i don't even want to think about trying to do anything with the tabs because that just makes my head spin to think about it. i know it's possible. i just. also know it would be a pain.

from there it's just. all varying degrees of stuff you can't do since proboards won't let you do radio buttons or checkboxes or anything with an onclick function or any kind of javascript or jquery in the post. i also don't think they'll let you add in any new code things (like i know i've used just <topper> for an element on a template cuz i'm lazy), but i wouldn't quote me on that, i've not tried in on proboards.

i forget most of the other stuff but. this is the link to the original tutorial, it will probably spark some more things since it works for both easy ubbc and for newclass.

Sorry if any of this is covered in Kuroya’s old tutorial – but better to have overlap than have something not get mentioned, yeah?

01 Main things I can think of are – as you said – keyframe animations. ‘+’ and ‘~’ (such as .class:hover ~ .class2) don’t work. I’m PRETTY sure nth child shenanigans don’t work, as in, I remember it not working, but I haven’t actually double checked in a hot second. Some specific tags that don’t work are toggles, inputs, the like. If you can find a way to convert HTML tabs to Proboards tabs, you may genuinely be inhuman, sdfkjlhfksjhf. I’ve also come to learn that table/tr/td tags can’t be given classes ([ td][attr=”class”,”NAME”] doesn’t seem to work for some reason, for example), and when a tag is given both a class and a styling, the styling has to be listed in the tag before the attr ([ div style=””][attr=”class”,”NAME”] rather than [ div ][attr=”class”,”NAME” style=””]), which is kind of a no-brainer, but I always put my ‘style’ after my ‘class’, so I usually have to manually swap that around when converting stuff, sdlfkjhdskfjh. That’s… all I can think of off the top of my head?

02 Not that I know of, sorry. ToT Proboards itself wouldn’t have it, but potentially the creator of the Style Tags plugin has it listed somewhere on Proboards Support? Wouldn’t hold your breath, though. TuT

03 I’d absolutely love to help test this out! The last converter that was around was promising but JUST glitchy enough that it couldn’t really be reliably used. Conversion’s a nice easy seven steps for me, but knocking it down to a matter of ‘tossing it in a converter’ is definitely a very nice idea. 👀

Kuroya tbqh I was already a few minutes into my attempt at compiling a list of tags that PB doesn't support before you reminded me that people can just... name tags whatever the heck they want. Thank you for saving both my time & my sanity ;o;

This is (hopefully) the complete list of HTML tags that PB does support (with the Style Tags plugin), if anyone wants to suggest changes:
["a", "b", "big", "blockquote", "br", "center", "code", "div", "em", "font", "h1", "h2", "h3", "h4", "h5", "h6", "hr", "i", "img", "li", "marquee", "ol", "p", "pre", "s", "span", "strike", "strong", "sub", "sup", "table", "tbody", "td", "th", "tr", "u", "ul"]

pharaoh leap now I really want to check if any of those selectors will work if they're tossed in the body of a newclass tag (like how keyframes have to be formatted)... And thank you for volunteering! ;u; I'll likely message you today or tomorrow with a link, though there's still some changes I'd like to make (tabs not being among them, alas -- there's just way too many ways to implement them in HTML) it's still functional for most things I've put through it.