HTML5 uses better markups that are not only easier to use but also is slightly better for SEO purposes, while its not amazing for SEO as Google yets to favor HTML5 websites it does better from Rich Markups and the ASIDE elements and Articles which helps Google establish what the page is about and maybe elements which have nothing to do with the page itself. With HTML4, XHTML etc it's not possible to inform Google that say a Blog roll or links to the right of the page are not anything to do with the actual article.
Think Frameworks Rather than CMS
I think beginner website designers are better of exploring the various frameworks available then using those to build a website in any CMS. In this day and age you want to be thinking of the Responsive Design, I recently wrote an article about how it can help small businesses and why all website designers should use responsive design. Mobile internet surfing is exploding at the moment and with 4G becoming more available and the fact that Mobiles are becoming faster, I strongly believe that all designers should be opting to use responsive design in all their projects. Not only does it give you edge over your competitors if they are failing to take this up, it also gives your customer better experiences for their visitors. Failing to learn responsive may lose your sales as well.
Hundreds of Quality Frameworks
As a beginner it'll be beneficial for you to take a look at various frameworks, most of these frameworks have been compiled for Joomla and Wordpress.. Once you know a framework its pretty easy to get it to work with a CMS or simply obtain a copy that has been made for the CMS that you require.
Here's a few good Responsive Frameworks / Boiler Plates
I personally prefer Zurb Foundation because its not to heavy and offers a really nice fluid grid, while a lot of other people like Bootstrap but I find it just way to heavy and the grid system it uses I dislike, but as I've said there are hundreds and the best one is the one that makes you to work fast and deliver the best results. Some people fight over which one is best but to be honest the best framework or boiler plate is one that allows you as the designer to make use off the most.
Frameworks / Boiler Plates for Various CMS
Github is a great place for looking for frameworks that have already been converted into a Joomla Theme or Wordpress Template, meaning that you save lots of time and you just focus on the DESIGN elements of the site and how it should function.
Don't Believe Everything You Read
As I've mentioned that in the HTML Compatibility section there is plenty of ways of overcoming compatibility issues. HTML5 works fine and most who claim its not good are those who are frightened of change.
If you do decide to opt to use HTML5 ensure you bookmark this site as It helped me tons making the move from HTML4 to HTML5: HTML5 Doctor
W3C Believes in HTML5
I also wrote another article a way back about W3C making plans to finish HTML5 standard, which is worth a read. They have completed HTML5 and now working towards HTML 5.1 so its fully supported and believed in which should give you even more confidence in making the move.
It's worth noting that I am a HTML5 fan boy but other than that there will become a point where you will need to make that change because the advantages of using HTML5 will outweigh the ones using older style of coding. Why not make the change while your mind is younger, people delaying something that is sooner than later become standard doesn't make sense to me.