It's true that knowing where the tag should be closed can be a tricky guesstimation at best in many circumstances. There's a simple way to decide where to put the closing tag when in doubt, though: just stick it in the last possible place to have it nest properly. Thus, in your example, the </i> would be placed just before the </b>, like so:

While this may not give you exactly what the original poster intended, it does help to get your code to validate properly.