I am curious if the clients with older web browsers will have any issue in viewing and using sites with the default html5 template.
http://www.w3schools.com/browsers/browsers_stats.asp
If you refer to IE6 than the answer is yes, but there is a solution. You can create a script (mod) to route to different template in older browsers OR suggest to switch to newer browser.
As Basara suggested w3chools usage percentage is very low.