Rule 1.1.8
Summary
This test consists in checking whether each <canvas> tag , carrying information, owns a textual alternative.
Business description
Criterion
Test
Description
Chaque image bitmap (balise
<canvas>) porteuse d’information, vérifie-t-elle une de ces conditions ?
- La balise
<canvas>possède une alternative textuelle- Un contenu alternatif est présent entre les balises
<canvas>et</canvas>- L’élément
<canvas>est immédiatement suivi d’un lien ou bouton adjacent permettant d’accéder à un contenu alternatif.- Un mécanisme permet à l’utilisateur de remplacer l’élément
<canvas>par un contenu alternatif.
Level
A
Technical description
Scope
Page
Decision level
Decidable with marker
Algorithm
Selection
Set1
All the <canvas> tags, not within a link and not identified as captcha (see Notes about captcha detection)
Set2
All the elements of Set1 identified as informative image by marker usage (see Notes for details about detection through marker)
Set3
All the elements of Set1 identified neither as informative image, nor as decorative image by marker usage (see Notes for details about detection through marker)
Process
Test1
For each element of Set2, check whether the tag owns a textual alternative (see Notes for details about textual alternative detection).
For each occurrence of false-result of Test1, raise a MessageA.
Test2
For each element of Set3, check whether the tag owns a textual alternative (see Notes for details about textual alternative detection).
For each occurrence of true-result of Test2, raise a MessageB.
For each occurrence of false-result of Test2, raise a MessageC.
Messages
MessageA
- code : CheckPresenceOfAlternativeMechanismForInformativeImage
- status: Pre-Qualified
- parameter : tag text,
"aria-label"attribute,"computed accessible name","src"attribute - present in source : yes
MessageB
- code : CheckNatureOfElementWithTextualAlternative
- status: Pre-Qualified
- parameter : tag text,
"aria-label"attribute,"computed accessible name","src"attribute - present in source : yes
MessageC
- code : CheckNatureOfElementWithoutTextualAlternative
- status: Pre-Qualified
- parameter : tag text,
"aria-label"attribute,"computed accessible name","src"attribute - present in source : yes
Analysis
Not Applicable
The page has no <canvas> (Set1 is empty)
Passed
All <canvas> tags are identified as informative, and have a textual alternative (Test1 returns true for all the
elements of Set2 and Set3 is empty).
Pre-qualified
In all other cases
Notes
Textual alternative detection
The textual alternative can be set by the presence of any the following elements :
- Text associated via the
aria-labelledbyWAI-ARIA attribute - Presence of an
aria-labelWAI-ARIA attribute - Presence of text within the tag
That order has to be respected to compute the textual alternative.
For instance, if an aria-label WAI-ARIA attribute and some text within the tag are both present, the content of the
aria-label WAI-ARIA attribute is considered as the textual alternative.
If none of these elements are present, two other conditions exist to set the textual alternative :
- A link, or a button is present just before or just after the element
- A mechanism exists to let the user replace the element by an alternative content.
Markers
Informative images markers are set through the INFORMATIVE_IMAGE_MARKER parameter.
Decorative images markers are set through the DECORATIVE_IMAGE_MARKER parameter.
The value(s) passed as marker(s) will be checked against the following attributes:
classidrole
Captcha detection
An element is identified as a CAPTCHA when the “captcha” occurrence is found :
- on one attribute of the element
- or within the text of the element
- or on one attribute of one parent of the element
- or within the text of one parent of the element
- or on one attribute of a sibling of the element
- or within the text of a sibling of the element