I vaguely remember years ago google.com omitting tags to strip file size and there being some mild controversy around it. Pretty sure it was before HTML 5 adaption and don't think back then the tags where optional, just that browsers try to interpret whatever mess is thrown at them.
This is an internal style guide, not external recommendations, as the title would suggest. Comments like these are really making me wary of the level of groupthink on here.
Now this makes sense while on my one page website such an omission would save the user like 10 bytes of extra data, when you add up the surface area of all Google properties that extra 10 bytes adds up across billions of page views.