Username Body Class appends stable identifiers to the HTML body element whenever a session is authenticated: a sanitized login slug (user-{login}) and numeric user ID (user-id-{id}). Logged-out visitors do not receive these tokens. The front end and wp-admin both reflect the classes, giving designers and support engineers a hook to tailor experiences per account—banner text, debug outlines, or admin density—without embedding PII in JavaScript globals.

Because login names appear in page source, treat this as semi-public metadata: avoid enabling on highly sensitive extranets if policy forbids disclosing login handles, even though they are often guessable from author archives.