Checks Reference

Horn implements 21 check modules validating 78 individual rules across the Matterhorn Protocol checkpoints. Each check targets specific PDF/UA-1 (and some PDF/UA-2) failure conditions.

Check modules

baseline — Checkpoint 01–31

Built-in pdf_oxide PDF/UA-1 structure validation. Provides foundational checks that other modules build on.

structure — Checkpoint 01, 02

Tagged PDF structure validation.

Rule	Description
01-003	/MarkInfo must exist with /Marked = true
01-004	/StructTreeRoot must exist and have children
02-001	/RoleMap entries must resolve to standard types; circular chains detected

content_stream — Checkpoint 01, 10, 31

Content stream analysis.

Rule	Description
01-001	All text operations must be inside BMC/BDC..EMC marked content sequences
01-002	Image XObject invocations (Do) must be inside marked content sequences
01-005	Artifact content must not be nested inside tagged content
31-025	Text operations must not reference the .notdef glyph (CID 0)

language — Checkpoint 02, 11

Natural language specification for text strings.

Rule	Description
02-001	Outline /Title text must have language context (catalog /Lang or struct-level /Lang)
02-002	Annotation /Contents text must have language context
02-003	Widget /TU (tooltip) text must have language context
02-004	XMP dc:title must have a real language (not just x-default) when no catalog /Lang
11-002	All /Lang values must be valid BCP 47 tags
11-005	Elements with /Alt text need language context
11-006	Elements with /ActualText need language context
11-007	Elements with /E (expansion text) need language context

version — Checkpoint 05

PDF/UA version identification and XMP extension schema.

Rule	Description
05-001	/Metadata stream must exist in catalog
05-002	pdfuaid:part value must match standard (1 for UA-1, 2 for UA-2); UA-2 requires PDF 2.0
05-003	pdfuaid:part identifier must be present
05-004	Extension schema for pdfuaid must be properly defined (correct URI and prefix)
05-005	No duplicate extension schema definitions for the PDF/UA namespace

metadata — Checkpoint 06

Document-level metadata validation.

Rule	Description
06-001	Document catalog must contain /Lang entry
06-002	XMP must contain pdfuaid:part identifier
06-003	ViewerPreferences/DisplayDocTitle must be true
06-004	XMP must contain dc:title

dict_entries — Checkpoint 07, 25

Dictionary-level validation and structural integrity.

Rule	Description
07-001	StructTreeRoot must contain /ParentTree; completeness validated
07-002	MarkInfo/Suspects must not be true
07-003	Non-standard structure types must have RoleMap entries
25-001	Reference XObjects (/Ref on Form XObjects) are forbidden

nesting — Checkpoint 09

Structure element parent-child rules.

Rule	Description
09-001	TR must be inside Table/THead/TBody/TFoot
09-004	TH/TD must be inside TR
09-006	Container child type rules: Table→TR/THead/TBody/TFoot/Caption; TR→TH/TD; L→LI/Caption; LI→Lbl/LBody; TOC→TOCI/TOC/Caption
—	Cardinality: at most one THead/TFoot per Table; THead/TFoot require TBody
—	Caption position: first or last for Table; first for TOC and List

images — Checkpoint 13

Figure accessibility.

Rule	Description
13-004	Figure elements must have /Alt or /ActualText
13-005	/Alt must not be empty

headings — Checkpoint 14

Heading hierarchy validation.

Rule	Description
14-002	First heading should be H1
14-003	Generic H headings must use nesting to convey hierarchy (no sibling H elements)
14-006	No skipped heading levels (e.g., H1 followed by H3)
14-007	Must not mix numbered (H1–H6) and generic (H) headings

tables — Checkpoint 15

Table structure and header association.

Rule	Description
15-002	Tables must contain TR with TH or TD children
15-003	Tables must have at least one TH header cell
15-004	TH cells must have /Scope (/Row, /Column, or /Both); invalid values flagged
15-005	Complex tables need /Headers, /Scope, or THead/TBody for header association
15-006	RowSpan/ColSpan must be valid positive integers within table dimensions

lists — Checkpoint 16

List structure validation.

Rule	Description
16-001	L must contain LI children
16-002	LI must contain Lbl or LBody
16-003	LBody structure validation

math — Checkpoint 17

Rule	Description
17-001	Formula elements must have alternative text (/Alt)

notes — Checkpoint 19

Rule	Description
19-001	Note elements must have /ID attribute
19-002	NoteRef links validation

optional_content — Checkpoint 20

Rule	Description
20-001	Optional content groups must have /Name
20-002	Default OCG configuration must be valid
20-003	/AS entry (auto-state) is forbidden

embedded_files — Checkpoint 21

Rule	Description
21-001	Embedded files must have /AF relationship
21-002	File specification must have /Desc

xfa — Checkpoint 25

Rule	Description
25-001	Document must not contain XFA form data

security — Checkpoint 26

Rule	Description
26-001	Encryption must not block assistive technology access
26-002	Security handler must allow content extraction for accessibility

annotations — Checkpoint 28

Annotation accessibility (page-level checks).

Rule	Description
28-001	Pages with annotations must have /Tabs = /S (structure order)
28-004	Link annotations must have /A (action) or /Dest (destination)
28-006	Annotations should have /Contents for accessible text
28-009	Form fields must have /T (field name) or /TU (tooltip)

annot_struct — Checkpoint 28

Annotation-to-structure-tree cross-validation.

Rule	Description
28-002	All annotations (except Popup/PrinterMark) must have OBJR in structure tree
28-003	Parent struct element type must match annotation subtype (Link→/Link, Widget→/Form)
28-005	Screen annotations must have /CT (content type) on media clip
28-006	Annotations under /Annot struct elements need /Contents or /Alt; zero-size (invisible) annotations exempted
28-007	TrapNet annotations forbidden; PrinterMark validation
28-008	FileAttachment must have /FS with /F and /UF
28-009	Form fields need /TU (tooltip) or /Alt on parent; zero-size (invisible) widgets exempted

fonts — Checkpoint 31

Font embedding, encoding, Unicode mapping, and font program validation. Uses ttf-parser for TrueType font introspection.

Rule	Description
31-001	All fonts must be embedded (FontFile/FontFile2/FontFile3)
31-002	CIDFontType2 must have /CIDToGIDMap (/Identity or stream)
31-003	CIDFont must have valid /CIDSystemInfo; CMap encoding validated; CIDFont Supplement must not exceed CMap Supplement
31-004	/CIDSet must be a valid stream when present
31-005	/Encoding must be a valid predefined name or dictionary; non-symbolic TrueType must have encoding
31-006	Font must have /ToUnicode CMap or standard encoding
31-007	ToUnicode CMap must not map to U+0000 (null), U+FFFE, or U+FEFF (noncharacters)

Severities

Each finding has a severity level:

Severity	Meaning
error	The PDF violates a PDF/UA-1 requirement
warning	Potential issue that may affect accessibility
info	Informational finding or best-practice suggestion

Check outcomes

Each check produces one of four outcomes:

Outcome	Meaning
Pass	The document satisfies this check
Fail	A violation was found (includes a message and optional location)
NeedsReview	The check cannot determine compliance automatically — manual review required
NotApplicable	The check does not apply to this document

Listing checks

Run horn list-checks to see all checks registered in your version of Horn:

horn list-checks