I'm not quite sure that I understand the question.
Yes, CSS3 and HTML5 are skills as well as PHP.
If you ask them to use Joomla, you're indeed asking them to use PHP because Joomla is a Content Management System, a software to handle content, already made by a company which allows you to personalize it as much as you want. Still, this company that created it used PHP to do so. So asking someone to use Joomla & PHP is redundant. PHP is the programming language that was used in Joomla.
Now, for your web pages, even if they use Joomla as an "engine" to handle that content, you can use any flavour of HTML and CSS you want. Asking them to use CSS3 and HTML5 is not unreasonable.