The Web Content Accessibility Guidelines (WCAG) include specific recommendations for how to make your site understandable by its visitors and assistive technologies they might use. The understandability principle is further divided into three guideline categories: readability, predictability, and input assistance.
WCAG 2.1 A/AA identifies two basic and essential success criteria for readability: WCAG success criteria 3.1.1 (Language of Page) and 3.1.2 (Language of Parts).
WCAG Success Criterion 3.1.1: Language of Page
Any assistive technology, like screen readers, that visitors to your site are using must be able to quickly identify the natural human language the page is written in. This makes sense if we consider that people from all over the world or speaking any number of languages may visit your website. In order for a screen reader to accurately understand and communicate information to somebody, it needs to know the correct language being used.
That’s why WCAG success criterion 3.1.1 (Language of Page) is so important. In order to be accessible, your website should specifically identify the language of each page in a manner that can be “programmatically determined” (i.e. a way that’s understandable to software such as screen readers).
To achieve this, designate the language of a web page using the HTML lang attribute at the top of the page’s source code. For example, if a page is written in English, then you would indicate that with this code snippet: <html lang="en">. Remembering to account for the page’s primary language in this simple way can help applications like screen readers understand and pronounce the content more accurately.
WCAG Success Criterion 3.1.2: Language of Parts
In addition to the page’s primary language, it’s also important to identify any changes of language within a page so that screen readers have the information they need to pronounce all the content on a page.
For example, maybe you have a page that is primarily in English but you have one module in Spanish to reach a different audience. Or, maybe you are inserting a commonly used phrase from another language, like déjà vu into an otherwise English sentence. In these examples, if the language of those parts of the page are not programmatically determined, the screen reader will attempt to read these foreign languages according to the English rules of pronunciation — depending on the content and its purpose, this may be hardly noticeable, a minor nuisance, or a major accessibility concern that renders the information incomprehensible.
In order to prevent these difficulties, the WCAG success criterion 3.1.2 (Language of Parts) requires websites to identify the language of any text that differs from the page’s main language. Screen readers can then pronounce these passages in the correct language. Like the Language of Page checkpoint, this is usually accomplished using the HTML lang attribute.
Additional WCAG Success Criteria for Readability
Your website usually isn’t expected to meet Level AAA of WCAG, the third and highest level of compliance, across the board. Still, being aware of Level AAA success criteria may enhance your accessibility efforts.
Language of Page and Language of Parts are Level A criteria in WCAG, and the only two of the six WCAG readability criteria below Level AAA. The four Level AAA success criteria help ensure that people have a way to understand unusual words, abbreviations are explained, pronunciation is clarified for words that could be ambiguous, and that alternative versions of content that don’t require advanced levels of reading are available if appropriate.