Represents the blocking attribute for an element.
The blocking attribute must have a value that is an unordered set of unique space-separated tokens,
each of which are possible blocking tokens.
The blocking attribute indicates that certain operations should be blocked on the fetching of an external resource.
The value is an unordered set of unique space-separated tokens, each of which are possible blocking tokens.
Represents the blocking attribute for an element. The blocking attribute must have a value that is an unordered set of unique space-separated tokens, each of which are possible blocking tokens.