How should UAs handle web authors setting dropEffect values?

[ragoulik]

The drag and drop spec specifies how the dropEffect value should be set by UAs for different events here: https://html.spec.whatwg.org/multipage/dnd.html#dndevents. However, the spec is not explicit about the following:

• Should UAs allow authors to set the dropEffect value to any valid value?
• If authors can set the dropEffect value, should UAs reflect that choice in the visual drop effect? For example, in dragenter event, if effectAllowed is copyMove and UA’s preferred value is copy but web author sets it to move, should this be reflected in attribute value?

[annevk]

On setting, if the new value is one of "none", "copy", "link", or "move", then the attribute's current value must be set to the new value. Other values must be ignored.

And then if you look further on in the processing model you'll find that there is some branching on the current value.