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XSLT-processor TODO

This document maps features from the W3C XSLT 1.0 Specification, W3C XSLT 2.0 Specification, and W3C XSLT 3.0 Specification against the current implementation. Items marked as unimplemented are identified from the specifications and/or the codebase.

VERSION SUPPORT: The processor currently targets XSLT 1.0 compliance with selective XSLT 3.0 features. XSLT 2.0+ features are marked with [XSLT 2.0] tags, and XSLT 3.0+ features are marked with [XSLT 3.0] tags. Most XSLT 2.0 features remain unimplemented. Several XSLT 3.0 features have been implemented, including accumulators, iterators, error handling, dynamic evaluation, and partial package/streaming infrastructure.

Unimplemented XSLT Elements (Section 2-7, B)

Partially Implemented or Incomplete Elements:

  • <xsl:import> - Basic support exists but template precedence rules may not be fully correct (Section 2.6.2, 5.5)
  • <xsl:number> - Currently implemented but may have incomplete support for all formatting options (Section 7.7)
    • Level attribute handling (single, multiple, any) - needs verification
    • Count/from patterns - needs verification
    • Number to string conversion attributes (format, lang, letter-value, grouping-separator, grouping-size) - partial support
  • <xsl:namespace-alias> - Namespace aliasing for result tree (Section 7.1.1)
  • <xsl:preserve-space> / <xsl:strip-space> - Whitespace handling is implemented but may have edge cases (Section 3.4)

XPath Functions (Section 12)

XSLT Additional Functions (Section 12):

  • document() - ✅ Implemented (basic)

    • Requires documentLoader callback in context for external document loading
    • Empty URI returns current document
    • Returns empty node-set if loading fails

  • key() - ✅ Implemented

    • Works with declared keys via <xsl:key> elements
    • Key indexing via context.keys map

  • format-number() - ✅ Implemented

    • JDK 1.1 DecimalFormat pattern syntax supported
    • Integration with <xsl:decimal-format> elements
    • Grouping separators, decimal symbols, etc.

  • generate-id() - ✅ Implemented

    • Uses hash-based ID generation
    • Same node = same ID within transformation
    • Starts with alphabetic character

  • unparsed-entity-uri() - ✅ Implemented (stub)

    • Requires unparsedEntities map in context
    • Returns empty string if entity not found
    • DTD parsing not available in JavaScript environments

  • system-property() - ✅ Implemented

    • Returns xsl:version, xsl:vendor, xsl:vendor-url
    • Custom properties via systemProperties in context

  • element-available() - ✅ Implemented

    • All 34 XSLT 1.0 elements supported
    • Works with or without xsl: prefix

  • function-available() - ✅ Implemented

    • Core XPath 1.0 functions (26 functions)
    • XSLT 1.0 additional functions (9 functions)
    • Custom functions (matches, ends-with, xml-to-json, json-to-xml)

  • current() - ✅ Implemented

    • Returns the current node being processed

Known Issues & Areas Needing Verification

Template precedence and priority

Need to verify complete correctness of:

  • Template conflict resolution (Section 5.5)
  • Import precedence (Section 2.6.2)
  • Template priority attribute (Section 5.3)

Specification Compliance Verification Needed

The following features are implemented but need complete specification compliance testing:

  • Modes (Section 5.7) - Template processing with different modes
  • Variables and Parameters (Section 11) - Scope and binding rules
  • Sorting (Section 10) - Sort key evaluation and language-dependent sorting
  • Patterns (Section 5.2) - Pattern matching against nodes
  • Literal result elements (Section 7.1.1) - Namespace handling and attribute value templates
  • Forwards-compatible processing (Section 2.5) - Version attribute handling for XSLT 2.0+ stylesheets

Major New Instructions/Declarations (not in XSLT 1.0):

  • <xsl:analyze-string> - Parse text against regular expressions (Section 15)

    • Requires regex engine support
    • Provides xsl:matching-substring and xsl:non-matching-substring children

  • <xsl:for-each-group> - Group sequences of nodes (Section 14)

    • Attributes: group-by, group-adjacent, group-starting-with, group-ending-with
    • Functions: current-group(), current-grouping-key()

  • <xsl:function> - Define stylesheet functions (Section 10.3)

    • User-defined functions callable from XPath expressions
    • Support for parameters and return types via as attribute
    • Override attribute for controlling function precedence

  • <xsl:import-schema> - Load XML Schema (Section 3.13)

    • Schema-aware XSLT processor feature
    • Provides type information for validation

  • <xsl:namespace> - Create namespace nodes (Section 11.7)

    • Computed namespace URI and prefix

  • <xsl:next-match> - Continue to next matching template (Section 6.6)

    • Similar to xsl:apply-imports but for next template match
    • Parameters passed via xsl:with-param

  • <xsl:perform-sort> - Sort a sequence (Section 13)

    • Returns sorted sequence instead of processing

  • <xsl:result-document> - Create multiple result documents (Section 19.1)

    • href attribute specifies output URI
    • Output formatting via serialization parameters
    • Supports validation and type attributes

  • <xsl:character-map> - Map characters on serialization (Section 20.1)

    • Maps Unicode characters to output strings
    • Alternative to disable-output-escaping
    • Contains xsl:output-character children

XSLT 2.0 Enhancements to Existing Instructions:

  • <xsl:apply-templates> - Enhanced with mode support

    • Can specify multiple modes with comma-separated list
    • #current pseudo-mode for continuing in current mode
    • #default pseudo-mode for default mode processing

  • <xsl:template> - Enhanced with multiple modes

    • mode attribute can have #all special value
    • Can apply to multiple modes simultaneously

  • <xsl:sort> - Enhanced with collation support (Section 13.1)

    • collation attribute for custom collation URI
    • stable attribute for stable sort behavior (yes/no)
    • data-type can be QName (not just "text" or "number")

  • <xsl:value-of> - Can now construct sequence

    • separator attribute to join multiple values
    • Can select sequences via select attribute

  • <xsl:copy>, <xsl:element>, <xsl:attribute> - Enhanced with:

    • validation attribute (strict/lax/preserve/strip)
    • type attribute for schema type validation
    • inherit-namespaces attribute
    • copy-namespaces attribute

  • <xsl:param> and <xsl:with-param> - Enhanced:

    • tunnel attribute for tunnel parameters
    • required attribute for stylesheet parameters (yes/no)
    • as attribute for sequence type declarations

  • <xsl:output> - Many new serialization options [XSLT 2.0]

    • byte-order-mark, escape-uri-attributes, normalization-form
    • undeclare-prefixes, use-character-maps
    • method can be custom QName (not just xml/html/text)

XSLT 2.0 New XPath Functions:

  • current-group() - Used within xsl:for-each-group (Section 14)

  • current-grouping-key() - Get current group key (Section 14)

  • unparsed-text() - Read external text file (Section 16.2)

    • Parallel to document() but for text files
    • Encoding parameter supported

  • unparsed-text-available() - Check if text file available (Section 16.2)

  • doc() and doc-available() - Load XML documents (Section 16.1)

    • XPath 2.0 functions referenced in XSLT context
    • More flexible than XSLT 1.0 document() function

  • collection() - Access document collections (Section 16.1)

    • Implementation-defined behavior

  • type-available() - Check if type name is available (Section 18.1.4)

    • Tests for schema type availability

  • Date/Time Functions - Comprehensive set (Section 16.5)

    • format-date(), format-time(), format-dateTime()
    • current-date(), current-time(), current-dateTime()
    • Language/calendar/country parameters for localization

XSLT 2.0 New Language Features:

  • Sequence Types - XPath 2.0 feature used in XSLT (Section 5.3, 9)

    • Used in as attribute of variables, parameters, functions, templates
    • Syntax: element(name), attribute(name), xs:string+, etc.

  • Attribute Value Templates - Enhanced expression support

    • Can use complex XPath 2.0 expressions inside {}
    • XPath 2.0 operators available

  • Mode System Enhanced - (Section 6)

    • mode attribute can list multiple modes
    • #default special mode
    • #current for inheriting current mode
    • Mode declarations on templates

  • Default Collation Attribute - default-collation on stylesheet (Section 3.6.1)

    • Specifies collation for string comparison
    • Used by keys, grouping, sorting

  • XPath Default Namespace - xpath-default-namespace attribute (Section 5.2)

    • Provides default namespace for unprefixed QNames in expressions
    • Different from XML default namespace

XSLT 2.0 Partial/Unimplemented Features:

  • Schema Awareness (Section 21.2) [XSLT 2.0 - Schema-Aware Processor]

    • Validation of constructed nodes
    • Type annotations in source and result trees
    • NOT IMPLEMENTED - requires XML Schema processor integration

  • Static Typing (Section 21) [XSLT 2.0 - Optional]

    • Optional feature for detecting type errors at compile time
    • NOT IMPLEMENTED

  • Backwards-Compatible Mode (Section 3.8) [XSLT 2.0]

    • Processing XSLT 1.0 stylesheets with XSLT 2.0 processor
    • Compatibility handling for version attribute
    • Fallback function binding

  • Forwards-Compatible Mode (Section 3.9) [XSLT 2.0]

    • Allows XSLT 2.0 stylesheets to run on XSLT 1.0 processors (with fallback)
    • Version attribute > 2.0 triggers special handling

Output Methods (Section 16)

XSLT 1.0 standard output methods are fully implemented:

  • ✓ XML output method (16.1)
  • ✓ HTML output method (16.2)
  • ✓ Text output method (16.3)
  • ✓ disable-output-escaping (16.4)

Extended output methods (XSLT 3.0):

  • ✓ JSON output method - ✅ IMPLEMENTED
  • ✓ Adaptive output method - ✅ IMPLEMENTED

All standard output attributes are implemented:

  • ✓ method, version, encoding, omit-xml-declaration, standalone
  • ✓ doctype-system, doctype-public, cdata-section-elements, indent, media-type

Validation & Error Handling

  • Forwards-compatible processing (Section 2.5) - Version attribute handling for XSLT 2.0+ stylesheets

    • Should gracefully ignore unknown elements/attributes from future XSLT versions
    • Needs verification for complete compliance

  • Schema validation (Section 17) - NOT IMPLEMENTED

    • DTD validation not available in JavaScript environments

XSLT 3.0 New Elements and Features [XSLT 3.0]

XSLT 3.0 (W3C Recommendation since June 2017) adds advanced features for package distribution, streaming, and enhanced text processing.

Major New Instructions [XSLT 3.0]:

  • <xsl:break> - Exit from xsl:iterate (Section 8.8.1) - NOT IMPLEMENTED

  • <xsl:iterate> - Iterate with state (Section 8.8) - ✅ IMPLEMENTED

    • More powerful than xsl:for-each, maintains iteration state
    • Attributes: select, as (sequence type)
    • Contains xsl:param for iteration parameters

  • <xsl:on-completion> - Complete iteration (Section 8.8.5) - ✅ IMPLEMENTED

    • Executed after xsl:iterate completes
    • Can access final iteration state

  • <xsl:try> / <xsl:catch> - Error handling (Section 20.5) - ✅ IMPLEMENTED

    • Structured error handling with pattern matching
    • Catch specific errors or all errors

  • <xsl:evaluate> - Dynamic XPath evaluation (Section 19.2) - ✅ IMPLEMENTED

    • Execute XPath expressions constructed at runtime
    • Supports context item and variable parameters

  • <xsl:on-empty> / <xsl:on-non-empty> - Conditional processing (Section 8.3) - ✅ IMPLEMENTED

    • Conditional processing within sequence-generating instructions

  • <xsl:package> - Package declaration [XSLT 3.0 - Packages] - ✅ PARTIAL

    • Root element for reusable XSLT packages
    • Infrastructure complete, full component resolution pending

  • <xsl:use-package> - Import package [XSLT 3.0 - Packages] - ✅ PARTIAL

    • Import XSLT package with version support
    • Infrastructure complete, full resolution pending

  • <xsl:override> - Override components [XSLT 3.0 - Packages] - ✅ PARTIAL

    • Override templates/functions from imported package
    • Infrastructure complete, full resolution pending

XSLT 3.0 Streaming Features [XSLT 3.0 - Streaming]:

  • <xsl:stream> - Process stream (Section 16) - ✅ PARTIAL

    • Infrastructure in place, actual document streaming pending

  • <xsl:merge> - Merge multiple sorted documents (Section 15) - ✅ PARTIAL

    • Infrastructure in place, full merge logic pending

  • <xsl:merge-source> - Merge source (Section 15.1) - ✅ PARTIAL

    • Infrastructure in place, full integration pending

  • <xsl:fork> - Fork processing [XSLT 3.0 - Streaming] - ✅ PARTIAL

    • Infrastructure in place, full fork logic pending

  • Streamable patterns - Pattern syntax for streaming - ✅ PARTIAL

    • Validator infrastructure exists, full streaming pending

XSLT 3.0 Enhancements to Existing Instructions:

  • <xsl:template> - New attributes [XSLT 3.0]

    • streamable attribute (yes/no) for streaming templates
    • cache attribute for caching results

  • <xsl:function> - Enhanced [XSLT 3.0]

    • cache attribute for function result caching
    • streamable attribute for streaming functions

  • <xsl:variable> / <xsl:param> - Enhanced [XSLT 3.0]

    • cache attribute for caching values
    • Can use streamable expressions

  • <xsl:copy-of> - Enhanced [XSLT 3.0]

    • New attributes for streaming context

  • <xsl:output> - New parameters [XSLT 3.0]

    • base-element for XHTML5 output
    • html-version for HTML5 support

  • <xsl:variable> - Enhanced initialization

    • Can use sequences without as attribute
    • Default behavior improved for common cases

XSLT 3.0 New XPath Functions:

Most XSLT 3.0 XPath functions are NOT IMPLEMENTED. The following are specific to XSLT 3.0:

  • map type operations (XPath 3.0)
    • map:new(), map:entry(), map:get(), map:put(), etc.
  • array type operations (XPath 3.0)
    • array:new(), array:size(), array:head(), etc.
  • parse-xml() and parse-xml-fragment()
  • serialize()
  • load-xslt()
  • function-lookup(), partial-apply()
  • current-group(), current-grouping-key()
  • unparsed-text(), unparsed-text-available()
  • Date/Time functions - format-date(), current-date(), etc.

XSLT 3.0 New Language Features:

  • Packages and Modules [XSLT 3.0 - Packages] - ✅ PARTIAL

    • Foundation complete, full component resolution pending
    • Package system infrastructure exists

  • Streaming mode [XSLT 3.0 - Streaming] - ✅ PARTIAL

    • Core architecture established
    • Actual document streaming pending

  • Higher-order functions [XSLT 3.0] - ✅ PARTIAL

    • Inline functions and named references implemented
    • Missing: function-lookup(), partial-apply()

  • Regular expression syntax [XSLT 3.0] - NOT IMPLEMENTED

    • XSD regex patterns
    • Named capture groups

XSLT 3.0 Output Methods [XSLT 3.0]:

  • html method enhancements - NOT IMPLEMENTED

    • HTML5 support with html-version parameter
    • base-element for correct link handling

  • xhtml method enhancements - NOT IMPLEMENTED

    • XHTML5 output capability

XSLT 3.0 Partial/Unimplemented Features:

  • Package system [XSLT 3.0 - Packages] - ✅ PARTIAL

    • Foundation complete, full component resolution pending

  • Streaming mode [XSLT 3.0 - Streaming] - ✅ PARTIAL

    • Core architecture established, actual document streaming pending

  • Higher-order functions [XSLT 3.0] - ✅ PARTIAL

  • Dynamic XSLT compilation [XSLT 3.0] - ✅ PARTIAL

    • xsl:evaluate for dynamic XPath implemented
    • Missing: load-xslt()

  • Schema Awareness [XSLT 3.0] - NOT IMPLEMENTED

    • Would require XML Schema processor integration

  • Static Typing [XSLT 3.0] - NOT IMPLEMENTED

Partially Tested Features

The following features need verification of complete specification compliance:

  • Modes (Section 5.7) - Template processing with different modes
  • Variables and Parameters (Section 11) - Scope and binding rules
  • Sorting (Section 10) - Sort key evaluation and language-dependent sorting
  • Patterns (Section 5.2) - Pattern matching against nodes
  • Literal result elements (Section 7.1.1) - Namespace handling and attribute value templates

Extension Mechanism (Section 14)

  • Extension elements - Framework exists but implementation depends on processor
  • Extension functions - Framework exists but implementation depends on processor
  • Error handling for missing extensions appears implemented

Statistics

XSLT 1.0 Coverage:

  • Implemented XSLT Elements: ~30+ out of 35+ elements in spec
  • Partially Implemented: ~5
  • Implemented XPath Core Functions: ~26+
  • XSLT Additional Functions: ✅ All 9 functions implemented
    • document(), key(), format-number(), generate-id(), current()
    • system-property(), element-available(), function-available()
    • unparsed-entity-uri() (stub - DTD not available in JS)

XSLT 2.0 Coverage:

  • New Instructions [XSLT 2.0]: 9 elements not in XSLT 1.0
    • Implemented: 0
    • Not Implemented: 9 (analyze-string, for-each-group, function, import-schema, namespace, next-match, perform-sort, result-document, character-map)
  • Enhanced Instructions [XSLT 2.0]: ~8+ existing instructions with new attributes/modes
    • Partial Support: Some enhancements present
  • New XPath Functions [XSLT 2.0]: 15+ functions
    • Not Implemented: 15+
  • New Language Features [XSLT 2.0]: Sequence types, enhanced templates, etc.
    • Minimal implementation: Basic type system exists

XSLT 3.0 Coverage:

  • New Instructions [XSLT 3.0]: 9 major new elements
    • Implemented: 5 (iterate, on-completion, try/catch, evaluate, on-empty/on-non-empty)
    • Partial: 3 (package, use-package, override)
    • Not Implemented: 1 (break)
  • Streaming Features [XSLT 3.0]: 5 elements - ✅ PARTIAL (infrastructure exists)
  • New XPath Functions [XSLT 3.0]: 20+ functions
    • Implemented: 2 (json-to-xml, xml-to-json)
    • Not Implemented: 18+
  • New Language Features [XSLT 3.0]:
    • Implemented: Accumulators, JSON functions, error handling, math functions
    • Partial: Packages, streaming, higher-order functions
    • Not Implemented: Maps/arrays as native types, schema awareness

Summary:

  • XSLT 1.0: ~90% functional (all major elements, all 9 XSLT functions)
  • XSLT 2.0: ~5% functional (mostly not implemented as separate layer)
  • XSLT 3.0: ~20% functional (select features: accumulators, iterators, error handling, dynamic evaluation, JSON, math functions, partial package/streaming infrastructure)

Implementation History

Version 4.x (2025-2026):

  • ✅ XSLT 3.0 accumulators (xsl:accumulator, xsl:accumulator-rule)
  • ✅ XSLT 3.0 iteration (xsl:iterate, xsl:on-completion)
  • ✅ XSLT 3.0 error handling (xsl:try, xsl:catch)
  • ✅ XSLT 3.0 dynamic evaluation (xsl:evaluate)
  • ✅ XSLT 3.0 conditional output (xsl:on-empty, xsl:on-non-empty)
  • ✅ XSLT 3.0 package system infrastructure (xsl:package, xsl:use-package, xsl:override)
  • ✅ XSLT 3.0 streaming infrastructure (xsl:stream, xsl:fork, xsl:merge)
  • ✅ XSLT 1.0 xsl:apply-imports element
  • ✅ JSON output method
  • ✅ Adaptive output method
  • ✅ concat() XPath function with node-set arguments
  • ✅ All XSLT 1.0 Section 12 functions
  • ✅ Versioned XPath parser API
  • ✅ XPath 3.0 math functions (all 14 functions)

Notes for Contributors