You pose some very interesting questions :)
First thing to remember is that the w3c validator (and others) only attempt to ensure that the page(s) are compliant to the standards for any given doc type. It says nothing about how the different browsers will render the site.
From what you have posted, and assuming the page(s) display in all the browsers you've tried, then you probably have nothing to worry about, at least for the time being.
If you change the attributes to make the pages html5 compliant and set the doc type to html5, then it'll pass the validation checks, but I suspect that some browsers will 'complain' or not render properly because they themselves are not yet html5 aware, and could possibly fall back to html1.1 (eek).
On the other hand, having html5 elements on the page(s) and not declaring them by the doc type could cause problems to existing browsers simply because they don't know how to handle the html5 elements.
In short, you are caught somewhere between a rock and a hard place :)
Ultimately, all websites will need to be updated for html5 compliance, but whether it is worth doing it yourself now, or whether you'll be better off waiting another year to see how things have evolved is a matter of debate.
Although it is 'nice' to have a site compliant with any given standard, I personally don't put much emphasis on them... Typically I'll only use them to help me find formatting errors (missing or badly formed tags, etc). Then regardless of any other compliance 'errors', I tend to ignore them and rely what I see with my own eyes when I view the site with several different web browsers. It'd be silly have a site with 100% compliance to a particular standard, if the browsers make a mess of things anyway.
As I said, you posed some very good questions, and my response should be considered as personal opinion only, and not to be taken as an authoritive answer.
Cheers
Rod.
"The nice thing about standards is that there are so many to choose from" - Andrew S. Tanenbaum
NOTE: We are sorry that Rod is no longer with us.
We are grateful for all his contributions to the Zen Cart community.
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