HTML 5 introduced the output element, which has a for
attribute defined as follows:
The for content attribute allows an explicit relationship to be made between the result of a calculation and the elements that represent the values that went into the calculation or that otherwise influenced the calculation. The for attribute, if specified, must contain a string consisting of an unordered set of unique space-separated tokens that are case-sensitive, each of which must have the value of an ID of an element in the same Document.
Are there any browsers or applications which use the value of this element, and if so for what? It's a neat idea but I'm not sure how it actually plays out.