As others have mentioned HTML5 is emerging and has not yet standardized. With experienced front-end coders there is criticism for the current HTML 5 working draft as it currently breaks semantics that have taken a long time to standardize, also some of the more advanced capabilities have spotty or poor support in some or all browsers. There are some facets of HTML 5 worth looking at, like a simplified compliant <!doctype html> and non-streaming native video and audio. It is worth understanding the issues surrounding HTML 5 so you can make informed choices about some (if any) html 5 capabilities you may use.
HTML 5 is not to be confused with CSS3, which is much more stable and useful. I encourage you to understand CSS including CSS3.
You should in this order learn:
- HTML 4.4
- HTML 5
- jQuery and YUI libraries
Learn the foundation of languages before using libraries. Also, as previously mentioned learn stable standards before working with emerging ones.