Web Accessibility Evaluation library written in Erlang.
The library tests web pages for conformance to the Web Content Accessibility Guidelines 2.0: WGAC 2.0.
It provides test of the level A of conformance for the HTML technology, allowing to:
- Validate the level A of conformance for HTML.
- Individually test the success criteria related to the level A of conformance over HTML.
- Individually test a subset of the HTML techniques considered as sufficient by the WCAG 2.0 to achieve the level A of conformance.
The library was designed in a way that eases the addition of new levels of conformance and technologies to test.
During the development process, Property-based testing was used to verify the HTML techniques, making use of the PropEr tool. An automated, random HTML generator was implemented as well during the testing phase.
a11y-checker, its Property-based tests and its automatic random HTML generator are licensed under GPLv3.
- Erlang/OTP and xmerl_scan parser.
- PropEr for the Property-based tests (PropEr installation instructions)
a11y-checker comes with an EMakefile you can use to compile and load the modules that compose the library by typing
erl -pa ebin
to open an Erlang shell and then
make:all([load]).
If you are interested in trying the property-based tests, remember to load or include in your Erlang classpath the PropEr modules:
l(proper).
l(proper_gen).
Once you have done this, you will have access to the APIs defined by the different modules. You can find examples of use in the next section.
a11y-checher is composed by several Erlang modules, each of one providing different APIs.
Those modules are:
Test levels of conformance and individual success criteria for different technologies.
conformance_level(LevelOfConformance, TechnologyList, Content, EntryType) -> boolean()
success_criterion(LevelOfConformance, TechnologyList, Content, EntryType) -> boolean()
functions of the last type look like success_criterion_n_n_n_description being the n the nomenclature given by the WCGA 2.0.
- LevelOfConformace, currently supported level is Level A.
- TechnologyList, currently sopported technology is HTML.
- Content, an string containing the HTML or a path to a file.
- EntryType = {string, file} depending on the nature of Content.
- True, if the content matches the conformance level or success criterion
- False, otherwise. Information of the failing HTML techniques and the failing HTML element are shown.
wcag_2:conformance_level(a, [html], "<html><head>Ejemplo</head><body><img>imagen</img></body></html>", string).
Output: (success messages are omitted for brevity)
Failing element: [{html,1},{body,2},{img,1}]
HTML H37: failed
Failing element: [{html,1},{body,2},{img,1}]
HTML H67: failed
Success Criterion 1.1.1: failed
.......
HTML H25: failed
Success Criterion 2.4.2: failed
.......
HTML H57: failed
Success Criterion 3.1.1: failed
.......
{validation_error,{error_missing_element_declaration_in_DTD,html}
HTML H88: failed
Success Criterion 4.1.1: failed
Success Criterion 4.1.2: failed
......
Level A of Conformance: denied
false
Explanation:
- Success criterion 1.1.1. fails because the image doesn't contain a textual alternative (H37), or a NULL alt attribute (H63).
- Success criterion 2.4.2. fails because the document doesn't contain a title (H25).
- *Success criterion 3.1.1. fails because the language attribute is not set in the html element (H57).
- Success criteria 4.1.1. and 4.1.2 fail because HTML is not used according to its specs, the DTD declaration is missing. (H88).
You can also provide a local file as argument:
wcag_2:conformance_level(a, [html], "file.html", file).
Finally, you can provide a URL as argument, but in that case make sure to start inets (and ssl) if needed:
inets:start().
ssl:start().
wcag_2:conformance_level(a, [html], "http://www.w3c.org", url).
Tests if a certain success criterion is met by the HTML of the page, for a certain level of conformance.
Its functionality is equivalent to the one provided by the wcag_2:success_criterion functions, because the technology currently supported is HTML.
Tests if a certain HTML Technique is correctly applied by the web content.
success_criterion_n(XmlElement) -> boolean()
named following the nomenclature used by the WCAG 2.0 (Hnum), plus a brief description of the technique.
-
XmlElement, structure representing the HTML document, obtained using the function scan() defined in the module is used:
scan (Html, type)
-
Html, an string containing the HTML or a path to a file.
-
type = {string, file} depending on the nature of Html.
- True, if the HTML applied correctly the technique
- False otherwise. Information of the failing HTML element is shown.
Testing [H37 HTML technique] (http://www.w3.org/TR/WCAG-TECHS/H37.html):
html_tech:h37_only_well_formed_images(html_tech:scan("<html><head>Example</head><body><img>Image</img></body></html>", string)).
Output:
false
The image doesn't contain a textual alternative.
There are two modules related to the property-based tests. To try them you have to get PropEr installed.
Automatic and random HTML generator.
Generate random (ocassionally valid and A-conformance level compliant) HTML string:
proper_gen:pick(html_gen:html()).
You can save the result permanently as a file:
{ok, F} = proper_gen:pick(html_gen:html()).
file:write_file("file.html", io_lib:fwrite("~p", [F])).
More information about proper_gen:pick and proper_gen.
Defines the properties that the HTML Techniques should follow.
Test the property related to (H37) over randomly generated HTML:
proper:quickcheck(html_prop:prop_well_formed_images()).
Proper looks for counterexamples (HTML code) where the property fails and provides an shrinked counterexample.
More information about PropEr here.
-
a11y-checker uses an XML parser: xmerl_scan to parse the HTML content. Hence, if your page is not well-formed a11y-checher will fail to parse the document.
-
a11y-checker currently supports HTML, if your page contains javascript code, parsing errors can happen when parsing the document using xmerl_scan.
-
Sometimes we have detected problems when parsing content of certain HTML attributes if they contain special characters.
-
The DTD's address specified in the HTML DOCTYPE declaration must be a local file, xmerl_scan doesn't have support to retrieve it from the web. You can use:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "src/html/data/DTD/xhtml1-transitional.dtd"> -
a11y-checker scans the HTML input up to 10 times when checking complete A conformance level. This should be optimised.