This is a general question which every developer faces, and in case the customer is techno savy then the customer will himself ask a site to be built in HTML5.0.
now the issue is should we go for it or not? is the HTML1.0 wrong?
Well there is nothing wrong with HTML1.0 if it is validated in W3C, and has no errors! There is no reason for you to move to HTML5.0. Unless of course if you want your site to be mobile compatible since HTML5.0 suits mobiles more because of features like scalable vector graphics(SVG).
I have seen that many SEO experts have advised to move on to HTML5.0 because it allows the developer to write h1 tags for each section head! This is not something special as per Matt cutts (google expert) a developer can mention h1 tags multiple times in HTML1.0 as well, it is only that one should spam it but one can certainly use it in a meaning full way for section headings which will cover the important keywords!
So bottom line there is no major rush for 5.0 BUT if you want to be update with technology and want to use features like SVG etc, then yes you should go for it, and by all means in few years HTML 5.0 will become a norm which can’t be avoided!
Finally where does this put the open source applications like magento, joomla, wordpress, prestashop etc?
WordPress already has HTML5.0 themes, but there aren’t many in magento, it is not that complicated to create a HTML5.0 theme but it is just that the developers are still not used to it that’s all.
Anyways welcome to HTML5.0!