Add accessibility section to autocomplete attribute page (mdn#43431)

* docs(HTML): add accessibility section to autocomplete attribute

Add accessibility considerations documenting how autocomplete values
help users with disabilities and satisfy WCAG 2.2 SC 1.3.5 (Identify
Input Purpose, Level AA).

Fixes mdn#37170
Continues the work from mdn#39088

* address pepelsbey review feedback

Add autocomplete="off" anti-pattern and invalid values subsection
to the accessibility section, as mentioned in issue mdn#37170.

* fix: split combined accessibility subsection into two separate headers

Separate "Avoid autocomplete='off' and invalid values" into two
distinct subsections per reviewer feedback: "Avoid disabling
autocomplete" and "Avoid invalid autocomplete values".

Author: kovan

files/en-us/web/html/reference/attributes/autocomplete/index.md

@@ -251,6 +251,20 @@ With {{htmlelement("input")}} and {{htmlelement("textarea")}}, the `webauthn` to
 - `webauthn`
   - : Passkeys generated by the [Web Authentication API](/en-US/docs/Web/API/Web_Authentication_API), as requested by a conditional {{domxref("CredentialsContainer.get()", "navigator.credentials.get()")}} call (i.e., one that includes `mediation: 'conditional'`). If included, this is the last token in the space-separated token list. See [Sign in with a passkey through form autofill](https://web.dev/articles/passkey-form-autofill) for more details.
 
+## Accessibility
+
+Using appropriate `autocomplete` values helps users with cognitive disabilities, motor impairments, and other conditions fill out forms more quickly and accurately by reducing the need to type and remember information. When the browser can identify the purpose of a form field through its `autocomplete` value, it can offer stored data such as names, addresses, and payment details, which benefits all users but is especially important for those who find manual data entry difficult.
+
+Providing valid autocomplete tokens also satisfies [WCAG 2.2 Success Criterion 1.3.5: Identify Input Purpose](https://www.w3.org/WAI/WCAG22/Understanding/identify-input-purpose) (Level AA), which requires that the purpose of input fields collecting user information can be programmatically determined. This enables user agents and assistive technologies to apply personalized presentations, such as displaying familiar icons next to fields, to help users understand and complete forms.
+
+### Avoid disabling autocomplete
+
+Setting `autocomplete="off"` prevents the browser from offering stored data for a field. While developers sometimes use this to prevent autofill for security reasons (such as one-time codes), it removes a feature that many users depend on. Users with cognitive disabilities may rely on autocomplete to recall personal information, and users with motor impairments benefit from reduced typing. Browsers may also ignore `autocomplete="off"` on login fields to support password managers.
+
+### Avoid invalid autocomplete values
+
+Using invalid or non-standard values (such as made-up strings to circumvent autofill) has a similar effect: the browser cannot match the field to any known purpose, so it cannot offer relevant suggestions. This also fails the WCAG requirement above, since the input purpose is no longer programmatically determinable. If you need to disable autofill for a specific field, consider using `autocomplete="off"` only where genuinely necessary (such as for CAPTCHA or one-time token fields) rather than applying it broadly across a form.
+
 ## Examples
 
 ```html