You should learn HTML5.
Now, which HTML5, that's another matter.
HTML5 - a framework designed to support innovation and foster the full potential the web has to offer.
Of course, for this ambitious project, many years have to pass before we can see this HTML5 properly supported by the significant browsers of the moment. Various parts of this so called HTML5 framework have different degrees of support in the modern browsers, and this is where the fast releases help. But it's also the biggest downfall since the differences in implementation move away from the standards.
They target the year 2014 for the full framework's specs to become the official recommendation. Probably by 2015-2016 we could see maybe half of it properly implemented by browser vendors. Maybe not. Who knows, by then, this view of HTML5 as a name for a framework it may became obsolete.
Another problem is that W3C has decided on a weird system to declare the specs final: it should exist at least two full implementations to make is so. Which creates a vicious circle: how could anyone fully implement specs that have never been completed by anyone yet, in order to be declared final?
In the meantime, another group, called WHATWG decided to go with HTML5 only in the classic sense, the HTML markup. Their specs came to a point where they'd simply call it HTML, to clear the air up of any confusion the W3C had cast on the HTML5 word.
The rest of the browsers have implemented the new semantics to a point where it's pretty much safe to leave behind HTML4 and use the new markup confidently.
To conclude, as far as the markup part is concerned, I'm already using the new semantic HTML5, and seeing that all, standard creators and browser vendors, have also concluded that 1999's HTML4.01 is way back in the past, my opinion is so should anyone starting up on web development today.