Drag and drop, accessibly

I need users to reorder <what>: <paste the current markup>

Implement it with:
- Mouse and touch dragging, with a clear drop indicator showing where
  it will land.
- A keyboard alternative — this is mandatory, not optional. Say how it
  works and how a user discovers it.
- Screen reader announcements for pick up, move and drop.
- The new order saved, with the request debounced so dragging three
  items isn't three round trips.
- What happens if the save fails — does the order revert?

Tell me whether a library is worth it here or whether this is small
enough to do directly.