Unfortunately it happens that browsers ship stub implementations of APIs that either do nothing, or throw exceptions. Such APIs appear to be there when doing feature detection, but can't actually be used.

Per Non-functional defined names imply partial_implementation these should be represented using partial_implementation, typically with notes explaining the situation.

The problem with this is that web developers really can't use these APIs, they don't work partially. APIs like this are worse than no support, as they confuse feature detection.

Examples of these:

• AbortController in Safari (Update AbortController Safari compatibility #3241)
• request.formData and response.formData in WebKit (Document Body.p.formData as always rejecting #6619)

Please add to the list or comment, if there are enough that would raise the priority of this issue.