Mapping of HTML select

Does anyone know the rationale behind mapping the HTML select element
differently based on its visual presentation?

 

According to the HTML AAM[1]:

 

* A select (with a multiple attribute or size attribute having value greater
than 1) is mapped as listbox.

 

* A select (with NO multiple attribute and NO size attribute having value
greater than 1) is mapped as combobox.

 

I suspect these mappings are a propagation of legacy desktop nomenclature.
Nonetheless, even if there is a historical desktop precedence, I do not
understand why a single-select list where only one option is visible at a
time is a combobox while a single select list where more than one option is
visible at a time is a listbox.

 

I have long understood that a primary purpose of ARIA roles is to
communicate interaction models to assistive technologies so that they may
assist users appropriately. In this case, the visual presentation of an
element, not its interaction model, is changing its role. A single select
with size of 1 and a single select with size of 4 both have a listbox
interaction model.

 

Telling the user that a select with size of 1 is a combobox when there is no
"box" (text field), seems very odd. I do not understand how it helps users.

 

The primary reason I am questioning the mapping at this time is because I am
trying to figure out how the authoring practices should tell authors when to
use the combobox role and when to use the listbox role. We are attempting to
make APG guidance consistent with typical native element behaviors across
browsers. In this case, however, it seems very confusing to recommend the
listbox role with one visual presentation and the combobox role with another
presentation when both have the same interaction model and functional
behaviors. This is especially confusing when we also refer authors to the
definition of combobox, which is the combination of a text input and a popup
element. A select with size of 1 does not have a text input or a separate
popup element.

 

Would there be any downsides to always mapping HTML select to listbox? It
would certainly make guiding authors much more straight forward and seems
that it could also help simplify experiences for screen reader users.

 

Matt

 

[1] http://rawgit.com/w3c/aria/master/html-aam/html-aam.html