Add semantic validation for CSS property names and value types

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The CSS parser performs syntactic parsing but lacks semantic validation, silently accepting typos like colr: red and invalid combinations like color: 10px.

Changes

Core Validation (CSSValidator)

• Property name validation with Levenshtein distance-based typo suggestions (distance ≤ 2)
• Value type checking against property requirements (color, length, percentage, etc.)
• Automatic allowlisting for custom properties (--*), vendor prefixes (-webkit-, -moz-, -ms-, -o-), and global keywords (inherit, initial, unset, revert, revert-layer)

Integration

• Modified AbstractCSSParser.handleProperty() to invoke validator when enabled
• Added setSemanticValidationEnabled(boolean) configuration API
• Validation disabled by default (opt-in for backward compatibility)

Data

• css-properties.txt resource file with 232 known CSS properties
• ValidationWarning class with INFO/WARNING/ERROR severity levels

Usage

CSSOMParser parser = new CSSOMParser();
parser.setSemanticValidationEnabled(true);
parser.setErrorHandler(new MyErrorHandler());
parser.parseStyleSheet(inputSource, null);

Parsing .example { colr: red; } now reports:

ERROR: Unknown CSS property 'colr'. Did you mean 'color'?

Test Coverage

• 14 unit tests for validator logic (property matching, Levenshtein distance, special cases)
• 8 integration tests with actual CSS parsing scenarios

Warning

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Original prompt

Add Semantic Validation to CSS Parser

Problem

The current CSS parser performs syntactic parsing but lacks semantic validation. This means:

• Unknown CSS properties are accepted without warnings (e.g., colr: red instead of color: red)
• Invalid property-value combinations pass validation (e.g., color: 10px)
• Type mismatches are not detected (e.g., width: red)
• No helpful suggestions when users make typos
• Browser compatibility issues are not flagged

Current behavior:

.example {
    colr: red;           /* Typo - silently accepted */
    color: 10px;         /* Wrong value type - accepted */
    background: inherit inherit;  /* Duplicate values - accepted */
    widht: 100%;         /* Typo - accepted */
}

All of the above parse successfully without any warnings, leading to CSS that won't work as expected.

Solution

Implement a comprehensive semantic validation layer that validates:

1. Property names against known CSS properties
2. Value types match property requirements
3. Property-value compatibility
4. Common typos with suggestions
5. Deprecated properties/values (optional)

---

Implementation Plan

1. Create CSSValidator Class

package org.htmlunit.cssparser.parser.validator;

import java.util.*;
import org.htmlunit.cssparser.parser.*;

/**
 * Validates CSS properties and values for semantic correctness.
 * Provides suggestions for common mistakes and typos.
 */
public class CSSValidator {
    
    private final Set<String> knownProperties;
    private final Map<String, Set<ValueType>> propertyValueTypes;
    private final boolean strict;
    
    public enum ValueType {
        COLOR,
        LENGTH,
        PERCENTAGE,
        NUMBER,
        INTEGER,
        URL,
        STRING,
        IDENTIFIER,
        TIME,
        ANGLE,
        FREQUENCY,
        RESOLUTION,
        FUNCTION,
        KEYWORD
    }
    
    public CSSValidator() {
        this(false);
    }
    
    public CSSValidator(boolean strict) {
        this.strict = strict;
        this.knownProperties = loadKnownProperties();
        this.propertyValueTypes = loadPropertyValueTypes();
    }
    
    /**
     * Validates a CSS property declaration.
     * 
     * @param property The property name
     * @param value The lexical unit value
     * @param locator The locator for error reporting
     * @throws CSSParseException if validation fails in strict mode
     * @return List of validation warnings (empty if valid)
     */
    public List<ValidationWarning> validateProperty(String property, LexicalUnit value, Locator locator) {
        List<ValidationWarning> warnings = new ArrayList<>();
        
        // Validate property name
        if (!isKnownProperty(property)) {
            String suggestion = findClosestPropertyMatch(property);
            String message = "Unknown CSS property '" + property + "'.";
            if (suggestion != null) {
                message += " Did you mean '" + suggestion + "'?";
            }
            
            ValidationWarning warning = new ValidationWarning(
                message, 
                locator, 
                ValidationWarning.Level.ERROR
            );
            warnings.add(warning);
            
            if (strict) {
                throw new CSSParseException(message, locator);
            }
            return warnings; // Don't validate value if property is unknown
        }
        
        // Validate value type
        if (value != null && !isValidValueForProperty(property, value)) {
            Set<ValueType> expectedTypes = propertyValueTypes.get(property.toLowerCase(Locale.ROOT));
            String message = "Invalid value type for property '" + property + "'. " +
                           "Expected: " + formatExpectedTypes(expectedTypes) +
                           ", got: " + getValueType(value);
            
            ValidationWarning warning = new ValidationWarning(
                message,
                locator,
                ValidationWarning.Level.WARNING
            );
            warnings.add(warning);
        }
        
        // Validate specific property rules
        warnings.addAll(validatePropertySpecificRules(property, value, locator));
        
        return warnings;
    }
    
    /**
     * Checks if a property name is a known CSS property.
     */
    public boolean isKnownProperty(String property) {
        if (property == null) {
            return false;
        }
        
        // Custom properties (CSS variables) are always valid
        if (property.startsWith("--")) {
            return true;
        }
        
        // Vendor prefixes are allowed
        if (property.startsWith("-webkit-") || 
            property.startsWith("-moz-") || 
            property.startsWith("-ms-") || 
            property.startsWith("-o-")) {
            return true;
        }
        
        return knownProperties.contains(property.toLowerCase(Locale.ROOT));...

</details>



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