Behavior if both webkitdirectory and multiple attributes are specified

[inexorabletash]

Raised by @triblondon over in w3ctag/design-reviews#215

What is the behavior if <input type=file webkitdirectory multiple> is used, i.e. if both webkitdirectory and multiple attributes are specified? Does one attribute override the other, or can multiple directories be selected?

The current behavior in Chrome is that webkitdirectory overrides multiple, and the latter has no effect. Only a single directory can be selected. I have not yet tested other implementations.

This should be specified somewhere. (Presumably in HTML eventually, but here for now.)

[inexorabletash]

Clicking on an input control generated via:
document.body.appendChild(Object.assign(document.createElement('input'), {type: 'file', webkitdirectory: true, multiple: true}))

• Chrome (70): multiple has no effect
• Firefox (63): multiple has no effect
• Edge (17): multiple has no effect
• Safari (11.1): multiple directories can be selected, the webkitRelativePath for each file includes the selected directory containing it (e.g. audio/1.mp3, images/2.png, etc)