The vCard text format
A vCard is a plain-text file: BEGIN:VCARD, a VERSION line choosing the dialect (2.1, 3.0 or 4.0), one line per property such as N, FN, TEL, EMAIL, ADR and ORG, then END:VCARD. Each property reads NAME;PARAMETERS:VALUE. The form maps to these properties, and the page writes CRLF line endings, UTF-8 text, and the photo folded in as base64. Everything runs in the browser, so the contact data never leaves the page.