a11y.label missing control

a11y.label-missing-control

Rule ID: a11y.label-missing-control Severity: ERROR Category: a11y Target Standards: WCAG 2.2 Success Criterion 1.3.1 (Info and Relationships - Level A), W3C WAI-ARIA Authoring Practices (Form Labeling), HTML5 Specification (Section 4.10.4 The label element)

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1. Overview & Core Invariant

Ensures label htmlFor attributes match an existing input control ID in the same document (WCAG 1.3.1)

Core Invariant:

"Every declaring an 'htmlFor' (or 'for') attribute must match the 'id' of an existing element in the same file."

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2. Technical Grounding & Engine Realities

HTML form labels use the htmlFor (or for) attribute to create a programmatic bond between text instructions and form controls.

When developers introduce typos (e.g. htmlFor="user_id" vs <input id="userId">) or delete inputs without updating labels:

1. Broken Click Targets: Clicking the label fails to focus or toggle the control, frustrating desktop and mobile touch users alike.
2. Screen Reader Disconnect: Screen readers read the label as isolated static text, leaving the actual input unannounced and unlabelled.
3. Linter False Negatives: Conventional linters only check that htmlFor exists as a string, completely ignoring that the target ID is missing.

Charites traverses the document AST symbol table to confirm that every declared htmlFor target actually exists.

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3. Vulnerability & Risk Taxonomy

Risk Vector	Severity	Impact
Broken Label Association	HIGH	Screen reader users receive unlabelled inputs; clicking labels fails to focus controls.
WCAG 1.3.1 Non-Compliance	HIGH	Critical Level A accessibility non-compliance.

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4. Non-Compliant Code Patterns (Bad Examples)

TSX (Typo in htmlFor target (user_id vs userId)):

<label htmlFor="user_id">ID Pengguna</label><input id="userId" className="border px-3 py-2" />

ASTRO (Label referencing non-existent control ID):

<label for="missing-input">Nama Lengkap</label><input id="fullname" class="border p-2" />

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5. Compliant Implementation Patterns (Good Examples)

TSX (Matching htmlFor and input id):

<label htmlFor="userId">ID Pengguna</label><input id="userId" className="border px-3 py-2" />

ASTRO (Accurate for reference matching input element):

<label for="fullname">Nama Lengkap</label><input id="fullname" class="border p-2" />

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6. Detection & Verification Pipeline (How The Rule Evaluates Code)

This a11y rule evaluates template markup and cross-references resolved design tokens for accessibility:

flowchart TD
    Node["AST Node (Astro / TSX element)"] --> Inspect["1. Inspect Interactive & Semantic Attributes"]
    Inspect --> BindCheck["2. Cross-Reference Form IDs / Labels / ARIA"]
    Inspect --> ContrastCheck["3. Query Token Graph for Foreground & Background Color"]
    ContrastCheck --> Luminance["4. Compute WCAG 2.2 Contrast Ratio"]
    BindCheck --> Invariant{"5. Validate Accessibility Invariant"}
    Luminance --> Invariant
    Invariant -- "Compliant" --> Safe["Pass"]
    Invariant -- "Violation" --> IgnoreCheck{"6. Check charites:ignore directive"}
    IgnoreCheck -- "Ignored" --> Safe
    IgnoreCheck -- "Not Ignored" --> Diag["7. Emit Diagnostic: a11y.label-missing-control"]

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Step-by-Step Evaluation:

1. AST Traversal: Inspects semantic elements, form controls, images, and landmark containers.
2. Structural & Attribute Invariant Check: Verifies accessible names, heading hierarchies, label/input bindings, and positive tabindex avoidance.
3. Token-Aware Contrast Verification: If analyzing color classes, queries token.Context to resolve foreground and background values, calculating relative luminance ratios.
4. Directive Suppression Check: Inspects preceding comments for charites:ignore a11y.label-missing-control.
5. Diagnostic Emission: Emits WCAG-grounded diagnostics for non-compliant patterns.

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7. Verification & Test Harness (How The Test Works: 1-SSOT Tri-Corpus)

This rule is rigorously tested and validated across the canonical 1-SSOT Tri-Corpus in tests/correctness/a11y.label-missing-control/:

flowchart TD
    subgraph GoldenCorpus ["1-SSOT Tri-Corpus Test Matrix for a11y.label-missing-control"]
        subgraph P ["Positive Corpus (tests/correctness/a11y.label-missing-control/positive/)"]
            P1["P1: Obvious Direct Violation"]
            P2["P2: Indirect / Variant Concatenation"]
            P3["P3: Helper / clsx / cn Wrapper"]
            P4["P4: Deeply Nested Elements"]
            P5["P5: Aliased Imports / Re-exports"]
        end
        subgraph N ["Negative Corpus (tests/correctness/a11y.label-missing-control/negative/)"]
            N1["N1: Valid Design Tokens"]
            N2["N2: Explicit charites:ignore Directive"]
            N3["N3: Third-Party / Vendor Components"]
            N4["N4: Clean Semantic HTML"]
            N5["N5: Untokenized Custom Values (Banana Test)"]
        end
        subgraph A ["Adversarial Corpus (tests/correctness/a11y.label-missing-control/adversarial/)"]
            A1["A1: Template Literal Interpolations"]
            A2["A2: Ternary Conditional Expressions"]
            A3["A3: Spread Properties & Dynamic Overrides"]
            A4["A4: Dynamic Object Class Syntax"]
            A5["A5: Shadowed Variable Identifiers"]
            A6["A6: Nested Closures & HOC Wrappers"]
            A7["A7: Obfuscated Classes & Cyclic Tokens"]
        end
    end

    P --> TestRunner["Automated Runner (rule_test.go)"]
    N --> TestRunner
    A --> TestRunner
    TestRunner --> Gates["Quality Gates: Zero Panic, Zero False-Positive, Zero Bypass"]

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• Positive Fixtures (P1-P5): Verified to trigger diagnostics at exact lines and column spans.
• Negative Fixtures (N1-N5): Verified to produce zero diagnostics on valid tokens and legitimate exemptions.
• Adversarial Fixtures (A1-A7): Verified to prevent evasion across dynamic expressions, string interpolations, and cyclic references.

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8. How to Suppress (Ignore Directives)

If this pattern is required for an intentional exception, suppress the diagnostic using the canonical Charites Rule ID:

<!-- charites:ignore a11y.label-missing-control intentional exception -->

// charites:ignore a11y.label-missing-control intentional exception

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9. Configuration Reference (charites.yaml)

rules:
  a11y.label-missing-control:
    severity: error # error | warn | info | off