>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.