Why it matters
Screen readers pick pronunciation rules from lang; without it text may be read in the wrong language. Translation tools also rely on it.
How to fix it
Set the page language on the root element.
Example
<html lang="en">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.
labelForm field has no labelselect-nameSelect element has no accessible nameinput-button-nameInput button has no accessible nameinput-image-altImage button has no alt texthtml-lang-valid<html lang> is not a valid language tagdocument-titlePage has no <title>landmark-one-mainPage has no <main> landmarkregionContent is outside landmark regions