We don't currently support the dropzone attribute [1] in the HTML5 drag and drop specification. Instead, we support a prefixed webkitdropzone attribute. Due to a bug in the code, the attribute only works on pages served from a file: origin.

We should fix the webkitdropzone behavior, ship it as dropzone, and then deprecate and remove webkitdropzone.

[1] https://html.spec.whatwg.org/multipage/interaction.html#the-dropzone-attribute