By now you should realize that the straightforward answer to how difficult it is to learn HTML5 would be that it is not difficult. This is dependent on the individual and what the user is actually trying to achieve.
HTML5 is not all about the tags. HTML5 supports built-in video and audio without the use of plug-ins. There is also a Canvas for displaying and creating sophisticated 2D graphics as well as drag-and-drop features for dynamic and improved Web application user interfaces.
HTML allows for storage of data on the local computer giving web applications the ability to function even when offline.
These features do tend to complicate things when it comes to HTML5. As it goes beyond the normal conventions that most web developers had become accustomed to. Unfortunately advanced features do not work in a vacuum or as simple elements. They require a third scripting to function.
The Canvas element with the 2D rendering context definitely has he steepest learning curve of any of the HTML5 elements.
So again you may ask. How difficult is it to learn HTML5? The answer is that it is relatively simple for basic usage generally on websites, and quite a challenge for advanced usage in relation to building web based applications as this comes with complexities.
The determining factor is your readiness and willingness to put time into reading the specification and following online tutorials and examples and within a few weeks you should be competent with your HTML5 skills.