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Validity map for <img> (Poll A, ISSUE-31, ISSUE-80, validation)

From: Leif Halvard Silli <xn--mlform-iua@xn--mlform-iua.no>
Date: Thu, 31 Mar 2011 04:12:43 +0200
To: HTMLWG WG <public-html@w3.org>
Message-ID: <20110331041243132485.ad9193fe@xn--mlform-iua.no>
This letter is a reference for my (not yet finished) reply in the poll 
[1][2] about how @role=presentation, @title, @aria-labelledby (as well 
as @aria-describedby and @aria-label) impact on the validity of an 
<img> element. 

Validity map: 

Hereby is offered a role based validity map, tested in ARIA supporting 
AT, which also takes into account that presentational images should not 
"show up" with author supplied or UA supplied fallback repair when/if 
images for some reason isn't displayed or enabled. (Thus validity of 
<img> should not only be about accessibility.) Non-empty @alt is taken 
literally: whitespace count as @alt, as specced by HTML5. In case of 
whitespace in @alt, AT will look for alternate text in @title or in 
@aria-* (according to my tests in VoiceOver).

* General:
  1. some attributes asked about in the poll impact the <img>'s
     role, which in turn sets validity requirements. 
  2. A basic requirement should be that @alt reflects the role:
      - role=presentation implies empty @alt, 
      - role=img implies non-empty @alt 
      - only if nothing else sets the role does @alt itself set it
     NOTE: VoiceOver works like that: It doesn't ignore <img> with
           empty alt in combination with non-empty @title. Such an
           image is also not considered presentational by sighted.
     NOTE: white-space according to HTML5 equals a non-empty @alt.
  3. an <img> which defaults to img role because nothing else
     implicitly or explicitly sets the role, is invalid:
      - with no-alt, the element defaults to img role, as long 
        as nothing else explicitly or implicitly sets its role.

* How role should be applied - overview. Value implies weight.
  2. Explicit role declaration
  1. Implicit role declaration
  0. Defaulting to role img (invalid)

* How role should be applied - in detail. Value implies weight.
  2. Explicit role declaration (always takes precedence)
     * presentation (role) is considered explicitly declared
       a. if parent has role:childrenArePresentational (ARIA 1.0)
       b. when <img> has @role=presentation
     * img (role) is explicitly declared 
       c. if <img> has @role=img and 2a isn't the case either;
  1. Implicit role declaration (in lack of explicit role)
     * img (role) is implied by the presence of:
       a. a non-empty @alt
       b. a non-empty @title
       c. a @longdesc of any value (depends on ISSUE-30)
       d. a non-empty aria-labelledby/aria-describedby/aria-label
       e. <img> + <figcaption>, as the only children of <figure>
       f. <img> as only child of semantic element:<a><dfn><h1> etc
       NOTE: For 1b to 1f, validator must ask for non-empty @alt.
     * presentation (role) is implied
       g. if @alt is empty and 1b,1c,1d,1e is not the case
  0. img role as result of default is invalid
     img role as result of default has these characteristics:
       * no explicit @role (2a,2b,2c)
       * no implicit role (1a,1b,1c,1d,1e,1f,1g)
       NOTE: adding @alt or @role or non-empty @title/@labelledby
             @describedby/@label gives the element an implicit or
             explicit role, whose validation rules then apply.

     NOTE: empty @title, @aria-labelledby, @aria-describedby
           or @aria-labelledby imply nothing and does thus
           not affect any role in any way.

* General validation rule:

  - When validator detects an explicit or implicit role, it must
    if needed, ask author for a corresponding @alt. EXAMPLES: 
    if @title is non-empty, there must be a non-empty alt. Or in
    case of explicit role="presentation", alt must be empty.
  - When validator detects an img role default, then it must ask
    author to add @alt (or give options/guidance what to do).

* Specific validation rules:

  # presentation (role), explicit or implicit:
    * empty @alt is required
    * a non-empty title would be invalid
    * any @longdesc (ISSUE-30) would be invalid
    NOTE, for explicit role=presentation:
      - @labelledby/@describedby/@label (any value) get disabled
        and thus does not need to be removed.
    NOTE for implicit presentation role:
      - implicit presentation cannot really be validated since any
        non-empty alt/title/labelledby/describedby/label/longdesc
        would change its role from presentation to img (tested
        in VoiceOver).
  # img (role) explicit or implicit:
    * non-empty alt is required
    * any value of title is valid
    * any value of -labelledby/-describedby/-label is valid

  # img role as result of default is always invalid
    * specialities, img role as result of default:
      - it has no @alt
      - labelledby/-describedby/-label/title if present, is empty

   Caveat: it really is up to ARIA whether empty -labelledby,
           -describedb or -label causes the element to be treated
           as having role img. But it seems logical that it has no
           impact. And VoiceOver seems to work that way.

Leif Halvard Silli
Received on Thursday, 31 March 2011 02:13:19 UTC

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