Most of the html I use is from 10 or 12 years ago. "Span?" We don't need no stinking span!
Anyway, I've gotten to where I know how to format using old style html when making mobi and epubs just fine. I was wondering if there's any advantage to learning the latest edition of html when making e-books? It just seems to me like there's more code involved, and most of it unnecessary. I'm of the mind that if it ain't broke, don't fix it, but I'm open to learning new things if there's any real benefit to it.
It really depends on what sort of formatting will best present your content and the platforms on which you'll be deploying.
For basic novels, or similar titles, you'll probably do just fine with HTML 3.x and never need to get into newer versions. However, if you've got content that would be better served with more advanced formatting features and you're deploying in ePub or Kindle Format 8 (whenever Amazon gets around to releasing the tools and docs for it) you'd definitely be well served to learn at least the subset of HTML 5 that is relevant to those tasks.
Ultimately it comes down to what tools you need to accomplish the task at hand and how much you're willing to invest to achieve the desired results. Personally I like to know as much about everything as possible and will take the opportunity to increase my knowledge even if it doesn't necessarily seem relevant at the moment.
You'll certainly be better prepared in the future if you do know at least the basic enhancements in HTML 5, and they're not that difficult to learn.
But if you don't have the time or interest it probably won't hurt you too much if you don't. I suspect that most basic novels will still work great with a dozen or so HTML tags even 10 years from now.