Or metadata for uses of attribute. We could have an attribute name convention for doing this. Meaning goes with attribute. Lots of poorly-specified attributes like 'role' and 'title.' Doesn't want to add to dataset class if specification will change. User-customizable site defaults.
Media types carry security warning.
Batch files, for example. By default in Internet Explorer everything is considered dangerous. Users can say to stop warning for all types except core list. A few want to discuss, no one wants to punt. Base spec requires support for UTF-8 but not complex mapping.