theme.nested opacity contrast

theme.nested-opacity-contrast

Rule ID: theme.nested-opacity-contrast Severity: WARN Category: theme Target Standards: WCAG 2.2 Success Criterion 1.4.3 (Contrast Minimum - 4.5:1), W3C DTCG State & Opacity Token Architecture, Hardware-Accelerated Compositing & Alpha Blending

---

1. Overview & Core Invariant

Detects nested opacity modifiers that compound to cause catastrophic text contrast degradation

Core Invariant:

"Containers with opacity or semi-transparent backgrounds must not enclose child elements with compounded opacity modifiers."

---

2. Technical Grounding & Engine Realities

When a parent container declares opacity (e.g. opacity-80 or bg-muted/40) and encloses child text or elements with another opacity modifier (e.g. text-foreground/50 or opacity-60), the browser multiplies effective alpha channels (0.8 × 0.5 = 0.40):

1. WCAG Contrast Catastrophe: Text that was theoretically compliant plummets below 2.5:1 contrast against the surface.
2. Inverted Washed-Out Appearance: Nested semi-transparency produces muddy, unreadable grey layers in dark mode.
3. Unpredictable Compositing: Nested opacity triggers extra GPU compositing passes and subpixel rendering degradation.

Charites enforces using pre-calibrated solid semantic tokens (e.g. text-muted-foreground instead of compounding text-foreground/50 over an opacity container).

---

3. Vulnerability & Risk Taxonomy

Risk Vector	Severity	Impact
Multiplicative Alpha Collapse	HIGH	Compounded opacity causes text contrast to fail WCAG AA 4.5:1 accessibility requirements.
Compositing Performance Overhead	LOW	Nested alpha layers force browser rasterization pipelines into multiple offscreen passes.

---

4. Non-Compliant Code Patterns (Bad Examples)

TSX (Container opacity compounded with child slash opacity in TSX):

<div className="bg-muted/40 opacity-80">
  <p className="text-foreground/50">Notice</p>
</div>

ASTRO (Nested opacity on parent and child text in Astro):

<section class="opacity-75">
  <span class="text-white/60">Subtle</span>
</section>

---

5. Compliant Implementation Patterns (Good Examples)

TSX (Using solid semantic container and pre-calibrated muted text token):

<div className="bg-muted">
  <p className="text-muted-foreground">Notice</p>
</div>

ASTRO (Solid background token and semantic foreground):

<section class="bg-card">
  <span class="text-foreground">Subtle</span>
</section>

---

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

This theme rule evaluates source templates against the project's design token graph:

flowchart TD
    Node["AST Node (Astro / TSX element)"] --> Extract["1. Extract Class Names (e.g. 'hover:bg-primary/10')"]
    Extract --> Strip["2. Strip Variants (hover:, dark:, sm:) -> 'bg-primary/10'"]
    Strip --> Split["3. Split Opacity Modifier -> Utility: 'bg-primary', Opacity: '/10'"]
    Split --> Convention["4. Query TokenConvention (Candidate: '--color-primary-light')"]
    Convention --> GraphQuery{"5. Check Token Graph (Does token exist in active scope?)"}
    GraphQuery -- "No (Banana Test)" --> Safe["Pass (Valid Custom / Untokenized Color)"]
    GraphQuery -- "Yes (Official Token Exists)" --> IgnoreCheck{"6. Check charites:ignore directive"}
    IgnoreCheck -- "Ignored" --> Safe
    IgnoreCheck -- "Not Ignored" --> Diag["7. Emit Diagnostic: theme.nested-opacity-contrast with Replacement Suggestion"]

Loading

Step-by-Step Evaluation:

1. AST Node Traversal: internal/analyzer streams JSX/Astro AST elements to the rule's Evaluate visitor.
2. Variant Normalization: Strips responsive (sm:, md:), interaction state (hover:, focus:), and theme (dark:) prefixes to isolate the core utility class.
3. Modifier Extraction: Parses utility segments and extracts slash opacity modifiers.
4. Token Convention Resolution: Consults the TokenConvention adapter to determine the official semantic design token replacement candidate.
5. Token Graph Verification (Banana Test): Queries token.Context to verify that the candidate token is declared in global.css or tokens.json within the element's scope. If not declared, the custom value is permitted without a false-positive diagnostic.
6. Directive Suppression Check: Inspects preceding AST comments for charites:ignore theme.nested-opacity-contrast.
7. Diagnostic Emission: Produces a structured diagnostic with line number, column span, and actionable replacement suggestion.

---

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/theme.nested-opacity-contrast/:

flowchart TD
    subgraph GoldenCorpus ["1-SSOT Tri-Corpus Test Matrix for theme.nested-opacity-contrast"]
        subgraph P ["Positive Corpus (tests/correctness/theme.nested-opacity-contrast/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/theme.nested-opacity-contrast/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/theme.nested-opacity-contrast/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"]

Loading

• 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.

---

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 theme.nested-opacity-contrast intentional exception -->

// charites:ignore theme.nested-opacity-contrast intentional exception

---

9. Configuration Reference (charites.yaml)

rules:
  theme.nested-opacity-contrast:
    severity: warn # error | warn | info | off