>>What I had in mind is to use something like
>>

outer text <span class="italic">text text text</span> more outer text

>>instead of the I tag, and similarly for other tags. Nothing changes in the interpretation of older text, just these classes (defined somewhere in your .css) would apply to text formatted using these buttons in new messages.
>
>Well, the same thing applies here. If the user would only put the starting tag and not the closing tag, we would end up in the same situation. Also, this would force the user to use a specific syntax. And, then, I would still have to validate all this.

Ah, but in all these cases the user doesn't see the syntax at all - the user selects a piece of text, clicks the button and the tag is applied to the text, and it's immediately rendered in HTML; tags are not visible at all. Only for those who are so inclined, some html source editing is still available but then the user is left with whatever they made - if it's unreadable, or won't display properly, they can only blame themselves. And that's how it works on practically any forum today.

I know that what we have here is a plain old textarea and what I'm describing is html editing, which is conceptually different in several ways. Mentioned this just because I thought it would be a cool feature to have, new or unaccustomed users would have an intuitive interface (i.e. what they're already used to) and you wouldn't have much work to do, as the code for it is widely available in various open source platforms.

Just something on my wish list, with very low priority.