See also #679

All form elements need a properly associated label. As per the WordPress accessibility standards, it must be an explicitly associated label (with for/id attributes, not wrapping the field).

The label can be visually hidden. Also, please consider the placeholder attribute is not a replacement for a label, see https://core.trac.wordpress.org/ticket/40331

Regarding the buttons, at the moment they don't have any text or label that can be announced by assistive technologies, they contain just SVG icons, see #528

These buttons need to be labeled, an aria-label attribute or a visually hidden text within the buttons would be appropriate.

when inserting a link:

when editing a link:

not sure what to do when editing a link, currently there's no form, just a not-editable link element with an empty href attribute. Waiting for development to complete for further review.