I've just tried validating my latest site and W3C is telling me:
The scheme attribute on the meta element is obsolete. Use only one scheme per field, or make the scheme declaration part of the value.
Seems that in HTML5 the 'scheme' attribute is now obsolete. Confirmed this on W3C and MDN.
So, what do we use instead? If anything? Do we just leave the scheme attribute off and everything just magically knows what the content is?