https://html.spec.whatwg.org/C/#tabindex-value lists a few types of elements that are "suggested" to be considered focusable. Within this list are the following:

• link elements that have an href attribute
• …
• Elements with a draggable attribute set, if that would enable the user agent to allow the user to begin a drag operations for those elements without the use of a pointing device

However, it doesn't appear that any browser actually implements these two things. The pretty comprehensive tabindex-focus-flag.html WPT does not test these two things either, though it tests everything else in the list.

I highly doubt the link element should be focusable, as it's used mostly for metadata. This clause was added in 457f1d5, which first suggested a list of things to consider focusable.

The draggable thing looks weird too: it was added in a2726c4 in response to this email by Aron Spohr, but no one seemed to have implemented it.