If anything, the loosening of the structural rules and nonsensical redundant tags set coding practices BACK a decade... which is why throwing some bloated scripted 'shiv' at it is stupid when the REAL answer is "Don't use HTML 5" -- problem solved.
Of course, since the site in question appears to be a wordpress skin, about 60% of the markup belongs in the trash in the first place... like all those stupid pointless bloated classes it throws on EVERYTHING for no good reason.
Here's a test for your "developer" -- if they don't know what's wrong with:
<li id="menu-item-22411" class="menu-item menu-item-type-taxonomy menu-item-object-category menu-item-22411">
It's time to find another developer.