SIA-R5`<html>` document elements have a valid `lang` attribute

Accessibility requirements

This rule tests conformance of the following accessibility requirements:

This rule checks that document elements have a lang attribute that is valid.

This rule applies to every lang attribute that is neither empty nor only ASCII whitespace and is declared on a document element which is an <html> element in the HTML namespace and in a top-level browsing context.

This rule makes no assumptions.

This rule has the following accessibility support concern:

<html> elements within <iframe> and <object> elements are not applicable as <iframe> and <object> elements create nested browsing contexts. However, as these elements are meant to provide a layer of isolation, the declared language of their parent browsing context will likely not be inherited, making it possible for non-valid lang attributes in nested browsing contexts to also cause accessibility issues.

This lang attribute has a value that is a valid language tag and therefore passes the rule:

<html lang="en"></html>

This lang attribute has a value that is not a valid language tag and therefore fails the rule:

<html lang="xyz"></html>

This lang attribute has a value that is empty and is therefore inapplicable:

<html lang=""></html>

This lang attribute has a value that is only ASCII whitespace and is therefore inapplicable:

<html lang=" "></html>