Implement error-message scanner rule (36b590)

[koddsson]

ACT Rule

• ACT Rule ID: 36b590
• Rule Name: Error message describes invalid form field value
• WCAG SC: 3.3.1 Error Identification (Level A)

What it checks

This rule checks that when a form field is in an error state, the error message is descriptive and programmatically associated with the field. It verifies that aria-describedby or aria-errormessage references point to elements that contain text describing the error.

Feasibility

DOM-only feasible. The check involves:

1. Finding form fields with aria-invalid="true"
2. Checking that aria-describedby or aria-errormessage references exist and point to elements with text content
3. Verifying the referenced error message elements are present in the DOM

No computed styles, layout, or browser APIs are required.

Notes

• Related to issue 35 W3C ACT rules have no corresponding scanner rule #350 (ACT rules triage)