TaxIdInput

One field for ein | ssn | itin | abn | gst | vat, each in its issuer's printed form, checked by the issuer's published algorithm where one exists and by format where none does. The result says which kind of check passed.

Scheme	Printed	Check
ein	12-3456789	Format only: nine digits. There's no check digit, and the IRS's valid-prefix list changes, so a stale copy here would reject a real number.
ssn	123-45-6789	Format: the area is never 000, 666 or 900–999, the group never 00, the serial never 0000.
itin	9XX-XX-XXXX	Format: a leading 9, with the fourth and fifth digits in 50–65, 70–88, 90–92 or 94–99.
abn	12 345 678 901	Checksum: subtract 1 from the first digit, apply the weights 10, 1, 3, 5 … 19, and the sum must be divisible by 89.
gst	123456789 RT0001	Canadian GST/HST: the nine-digit business number must pass Luhn, followed by the RT program account.
vat	DE123456789	Format by country prefix only (EU member states, XI, GB). An unknown prefix is "not checked", not invalid.

A pass is not a verification. A well-formed number may never have been issued. Registry lookups (IRS TIN matchingPayablesTIN matchingThe IRS e-Services check of a payee's name and taxpayer identification number against IRS records, open to payers who file information returns. It says whether the pair matches, not what the right number is, VIES, HMRC, ABN Lookup) belong to the consumer's backend and come back through error.

Masked whenever it isn't focused. Every character but the last four is a bullet in the DOM, so a screenshot, a screen share or a copied selection carries the mask, not the number. A VAT number keeps its country prefix, since the prefix isn't the secret. Focus shows the whole number, and formatting applies on blur only, because inserting a hyphen mid-typing moves the caret. With name, a hidden input posts the real value. The visible field never does.

Never logged: no console output, no data attribute, autoComplete="off", no spellcheck. type="text" with the scheme's inputMode, and paste is never blocked.

Validation timing: "incomplete" waits until the field is left. A wrong number at full length says so at once. The message line is reserved, so a result arriving doesn't move the form. It crossfades in and never shakes.

Accessibility: a real <label>. aria-invalid and aria-describedby point at the message. At rest the masked field is described as "SSN hidden, ending 6789. Focus the field to show it."