MyBB Community Forums

I have 2 questions:

1. Is it okay to use HTML in forum descriptions? Can I for example add <i> </i> or some css styling there?
2. I want some part of it to have css styling, should I go with <span style="...."> ?

Yes, it is ok to use it. Even MyBB uses it for some forums desription like "Security Management and Support" and so.

And yes, if it has few styles, it's easier to use that span.

But you can also use something like <span class="...."> and make that different for each theme in css

Please note that if you are using class, you will need to have style for that class in css stylesheet of each theme or it will not work for some of them