A missing point in the HTML5 spec

To the attention of the HTML5 specification authors and whoevere interested.
While tracking a bug in another project (Validator.w3.org/nu) I noticed
that the attribute "inputmode" has been inserted in the official spec. It
is listed in the <input> element (a table specifies on which input states
it applies) and in the <textarea> element lists of allowed attributes.
However, differently from any other attribute in the spec, it has no prose
to explain what it is needed for, or what kind of attribute it is
(enumerated), or its possible values.
Now spec should state whether @inputmode is actually allowed and in what
states and functions, or remove it because it is quite a problem for
authors.
Thank you in advance.