Why it matters
Screen readers and translation tools ignore an invalid lang value and fall back to the user's default language, mispronouncing the content.
How to fix it
Use a valid BCP 47 language tag such as en, en-GB or pt-BR.
Example
<html lang="en-GB">References
More accessibility rules
void a11y reports 48 rules in this category. WCAG 2.2 AA through axe-core, plus checks axe can't do alone: visible keyboard focus, 24/44px target size on mobile, 320px reflow and disabled zoom.
select-nameSelect element has no accessible nameinput-button-nameInput button has no accessible nameinput-image-altImage button has no alt texthtml-has-lang<html> has no lang attributedocument-titlePage has no <title>landmark-one-mainPage has no <main> landmarkregionContent is outside landmark regionslandmark-uniqueLandmarks are not uniquely identifiable