Mapbox Style Quality Skill

This skill provides expert guidance on ensuring Mapbox style quality through validation, accessibility, and optimization tools.

When to Use Quality Tools

Pre-Production Checklist

Before deploying any Mapbox style to production:

1. Validate all expressions - Catch syntax errors before runtime
2. Check color contrast - Ensure text is readable (WCAG compliance)
3. Validate GeoJSON sources - Ensure data integrity
4. Optimize style - Reduce file size and improve performance
5. Compare versions - Understand what changed
6. Remove empty layers - Delete layers with no visible paint properties as a final cleanup step
7. Simplify redundant boolean expressions - Clean up filters with unnecessary boolean logic (e.g., ["all", expr] → expr, ["any", false, expr] → expr)

During Development

When adding GeoJSON data:

• Always validate external GeoJSON with validate_geojson_tool before using as a source

When writing expressions:

• Validate expressions with validate_expression_tool as you write them
• Catch type mismatches early (e.g., using string operator on number)
• Verify operator availability in your Mapbox GL JS version
• Test expressions with expected data types

When styling text/labels:

• Check foreground/background contrast with check_color_contrast_tool
• Aim for WCAG AA minimum (4.5:1 for normal text, 3:1 for large text)
• Use AAA standard (7:1 for normal text) for better accessibility
• Consider different background scenarios (map tiles, overlays)

Before Committing Changes

Compare style versions:

• Use compare_styles_tool to generate a diff report
• Review all layer changes, source modifications, and expression updates
• Understand the impact of your changes
• Document significant changes in commit messages

Before Deployment

Optimize the style:

• Run optimize_style_tool to reduce file size
• Remove unused sources that reference deleted layers
• Eliminate duplicate layers with identical properties
• Simplify redundant boolean expressions in filters (e.g., collapse ["all", expr] to expr, remove tautological conditions)
• Remove empty layers (layers with no visible paint properties) as a final cleanup step

Validation Best Practices

GeoJSON Validation

Always validate when:

• Loading GeoJSON from user uploads
• Fetching GeoJSON from external APIs
• Processing GeoJSON from third-party sources
• Converting between data formats

Common GeoJSON errors:

• Invalid coordinate ranges (longitude > 180 or < -180)
• Unclosed polygon rings (first and last coordinates must match)
• Wrong coordinate order (should be [longitude, latitude], not [latitude, longitude])
• Missing required properties (type, coordinates, geometry)
• Invalid geometry types or nesting

Example workflow:

1. Receive GeoJSON data
2. Validate with validate_geojson_tool
3. If valid: Add as source to style
4. If invalid: Fix errors, re-validate

Expression Validation

Validate expressions for:

• Filter conditions (filter property on layers)
• Data-driven styling (paint and layout properties)
• Feature state expressions
• Dynamic property calculations

Common expression errors:

• Type mismatches (string operators on numbers)
• Invalid operator names or wrong syntax
• Wrong number of arguments for operators
• Nested expression errors
• Using unavailable operators for your GL JS version

Prevention strategies:

• Validate as you write expressions, not at runtime
• Test expressions with representative data
• Use type checking (expectedType parameter)
• Validate in context (layer, filter, paint, layout)

Accessibility Validation

WCAG Levels:

• AA (minimum): 4.5:1 for normal text, 3:1 for large text
• AAA (enhanced): 7:1 for normal text, 4.5:1 for large text

Text size categories:

• Normal: < 18pt or < 14pt bold
• Large: ≥ 18pt or ≥ 14pt bold

Common scenarios to check:

• Text labels on map tiles
• POI labels with background colors
• Custom markers with text
• UI overlays on maps
• Legend text and symbols
• Attribution text

Testing strategy:

• Test against both light and dark map tiles
• Consider overlay backgrounds (popups, modals)
• Test in different lighting conditions (mobile outdoor use)
• Verify contrast at different zoom levels

Quality Workflow Examples

Basic Quality Check

1. Validate expressions in style
2. Check color contrast for text layers
3. Optimize if needed

Full Pre-Production Workflow

1. Validate all GeoJSON sources
2. Validate all expressions (filters, paint, layout)
3. Check color contrast for all text layers
4. Compare with previous production version
5. Optimize style
6. Test optimized style
7. Deploy

Troubleshooting Workflow

1. Compare working vs. broken style
2. Identify differences
3. Validate suspicious expressions
4. Check GeoJSON data if source-related
5. Verify color contrast if visibility issue

Common Issues and Solutions

Runtime Expression Errors

Problem: Map throws expression errors at runtime Solution: Validate expressions with validate_expression_tool during development Prevention: Add expression validation to pre-commit hooks or CI/CD

Poor Text Readability

Problem: Text labels are hard to read on map Solution: Check contrast with check_color_contrast_tool, adjust colors to meet WCAG AA Prevention: Test text on both light and dark backgrounds, check at different zoom levels

Large Style File Size

Problem: Style takes long to load or transfer Solution: Run optimize_style_tool to remove redundancies and simplify Prevention: Regularly optimize during development, remove unused sources immediately

Invalid GeoJSON Source

Problem: GeoJSON source fails to load or render Solution: Validate with validate_geojson_tool, fix coordinate issues, verify structure Prevention: Validate all external GeoJSON before adding to style

Unexpected Style Changes

Problem: Style changed but unsure what modified Solution: Use compare_styles_tool to generate diff report Prevention: Compare before/after for all significant changes, document modifications

Tool Quick Reference

Reference Files

For detailed guidance on specific topics, load the relevant reference:

• references/optimization.md — Optimization types, strategies, recommended order, and maintenance best practices
• references/comparison.md — Style comparison workflows, ignoreMetadata usage, and refactoring workflow
• references/ci-integration.md — Git pre-commit hooks, CI/CD pipeline steps, and code review checklist

Load instruction: Read the reference file when the user needs in-depth guidance on that topic.

Additional Resources

• Mapbox Style Specification
• WCAG 2.1 Guidelines
• GeoJSON Specification (RFC 7946)
• Mapbox Expression Reference