EPUBCheck messages explained

EPUBCheck reports every problem in an EPUB as a code such as RSC-005 or OPF-014 with a severity. This reference lists all 246 messages of EPUBCheck 5.4.0 with their official text, and explains causes and fixes for 189 of them.

Updated: Nov 4, 2026

Severities

Fatal error: EPUBCheck could not continue checking this part of the EPUB. The file is invalid.
Error: The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.
Warning: The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
Usage: Informative only and hidden by default: it appears when EPUBCheck runs with -u (--usage). It does not make the EPUB invalid.
Info: Informative only. It does not make the EPUB invalid.
Suppressed: EPUBCheck does not report this message by default. It only appears if a custom message file (-c) changes its severity.

PKG: Package and ZIP container

The EPUB file itself: the ZIP archive, the mimetype file, file names and extensions.

Code	Default severity	Message
`PKG-001`	Warning	Validating the EPUB against version … but detected version ….
`PKG-003`	Error	Unable to read EPUB file header. This is likely a corrupted EPUB file.
`PKG-004`	Fatal error	Corrupted EPUB ZIP header.
`PKG-005`	Error	The mimetype file has an extra field of length …. The use of the extra field feature of the ZIP format is not permitted for the mimetype file.
`PKG-006`	Error	Mimetype file entry is missing or is not the first file in the archive.
`PKG-007`	Error	The content of the mimetype file must be the string "application/epub+zip"…
`PKG-008`	Fatal error	Unable to read file "…".
`PKG-009`	Error	The file name "…" contains characters that are not allowed in OCF file names: ….
`PKG-010`	Warning	The file name "…" contains spaces, which may create interoperability issues with older reading systems.
`PKG-011`	Error	The file name "…" is not allowed to end with ".".
`PKG-012`	Usage	The file name "…" contains non-ASCII characters, which might create interoperability issues with older reading systems.
`PKG-013`	Error	The EPUB file includes multiple OPS renditions.
`PKG-014`	Warning	The EPUB contains empty directory "…".
`PKG-015`	Fatal error	Unable to read EPUB contents: …
`PKG-016`	Warning	Use only lowercase characters for the EPUB file extension for maximum compatibility.
`PKG-017`	Warning	Uncommon EPUB file extension.
`PKG-018`	Fatal error	The EPUB file could not be found.
`PKG-020`	Error	OPF file "…" could not be found.
`PKG-021`	Error	Corrupted image file encountered.
`PKG-022`	Warning	Wrong file extension for image. The image is a "…" file but has the file extension "…".
`PKG-023`	Usage	Validating the EPUB against version 2.0, default validation profile will be used.
`PKG-024`	Usage	Uncommon EPUB file extension.
`PKG-025`	Error	Publication resource must not be located in the META-INF directory
`PKG-026`	Error	Obfuscated resource must be a Font Core Media Type (was declared as "…" in "…").
`PKG-027`	Fatal error	Could not extract EPUB ZIP content, probably due to file names not encoded in UTF-8.

RSC: Resources and references

Files and links between them: missing files, broken references, URLs, encodings and XML parsing.

Code	Default severity	Message
`RSC-001`	Error	File "…" could not be found.
`RSC-002`	Fatal error	Required META-INF/container.xml resource could not be found.
`RSC-003`	Error	No rootfile tag with media type "application/oebps-package+xml" was found in the container.
`RSC-004`	Info	File "…" is encrypted, its content will not be checked.
`RSC-005`	Error	Error while parsing file: …
`RSC-006`	Error	Remote resource reference is not allowed in this context; resource "…" must be located in the EPUB container.
`RSC-006b`	Usage	Resource "…" is located outside the EPUB Container; please check the resource is retrieved in scripted content.
`RSC-007`	Error	Referenced resource "…" could not be found in the EPUB.
`RSC-007w`	Warning	Referenced resource "…" could not be found in the EPUB.
`RSC-008`	Error	Referenced resource "…" is not declared in the OPF manifest.
`RSC-009`	Warning	A non-SVG image resource should not be defined with a fragment identifier.
`RSC-010`	Error	Reference to non-standard resource type found.
`RSC-011`	Error	Found a reference to a resource that is not a spine item.
`RSC-012`	Error	Fragment identifier is not defined.
`RSC-013`	Usage	Fragment identifier is used in a reference to a stylesheet resource.
`RSC-014`	Usage	Fragment identifier defines an incompatible resource type.
`RSC-015`	Usage	A fragment identifier is required for svg use tag references.
`RSC-016`	Fatal error	Fatal Error while parsing file: …
`RSC-017`	Warning	Warning while parsing file: …
`RSC-019`	Warning	EPUBs with Multiple Renditions should contain a META-INF/metadata.xml file.
`RSC-020`	Error	"…" is not a valid URL (…)
`RSC-021`	Error	A Search Key Map Document must point to Content Documents ("…" was not found in the spine).
`RSC-022`	Info	Cannot check image details (requires Java version 7 or higher).
`RSC-024`	Usage	Informative parsing warning: …
`RSC-025`	Usage	Informative parsing error: …
`RSC-026`	Error	URL "…" leaks outside the container (it is not a valid-relative-ocf-URL-with-fragment string)
`RSC-027`	Warning	XML document is encoded in UTF-16. It should be encoded in UTF-8 instead.
`RSC-028`	Error	XML documents must be encoded in UTF-8, but … was detected.
`RSC-029`	Error	Data URL is not allowed in this context.
`RSC-030`	Error	File URLs are not allowed in EPUB, but found "…".
`RSC-031`	Warning	Remote resource references should use HTTPS, but found "…".
`RSC-032`	Error	Fallback must be provided for foreign resources, but found none for resource "…" of type "…".
`RSC-033`	Error	Relative URL strings must not have a query component, but found one in "…".
`RSC-034`	Error	Scripts must have a JavaScript media type.
`RSC-035`	Usage	"…" is a legacy media type for JavaScript. Consider using 'text/javascript' instead.
`RSC-036`	Usage	…

OPF: Package document (OPF)

The package document: metadata, manifest, spine, properties and fallbacks.

Code	Default severity	Message
`OPF-001`	Error	There was an error when parsing the EPUB version: ….
`OPF-002`	Fatal error	The OPF file "…" was not found in the EPUB.
`OPF-003`	Usage	Item "…" exists in the EPUB, but is not declared in the OPF manifest.
`OPF-004`	Warning	Invalid prefix declaration: leading or trailing whitespace is not allowed.
`OPF-004a`	Error	Invalid prefix declaration: found empty prefix.
`OPF-004b`	Error	Invalid prefix "…": must be a valid non-colonized name (aka "NCName").
`OPF-004c`	Error	Invalid prefix declaration: prefix "…" must be immediately followed by a colon character (":").
`OPF-004d`	Error	Invalid prefix declaration: prefix "…" must be separated by its URI with a space.
`OPF-004e`	Warning	Invalid prefix declaration: found illegal whitespace between prefix and URI.
`OPF-004f`	Warning	Invalid prefix declaration: found illegal whitespace between prefix mappings.
`OPF-005`	Error	Invalid prefix declaration: URI for prefix "…" doesn’t exist.
`OPF-006`	Error	Invalid prefix declaration: URI "…" is not a valid URI.
`OPF-007`	Warning	Re-declaration of reserved prefix "…".
`OPF-007a`	Error	Invalid prefix mapping: prefix "_" must not be declared.
`OPF-007b`	Error	Invalid prefix mapping: default vocabulary "…" must not be re-declared.
`OPF-007c`	Error	Invalid prefix mapping: a prefix to the Dublin Core /elements/1.1/ namespace must not be declared
`OPF-010`	Error	Error resolving reference: "…".
`OPF-011`	Error	itemref can’t have both page-spread-right and page-spread-left properties.
`OPF-012`	Error	Item property "…" is not defined for media type "…".
`OPF-013`	Warning	Resource "…" is declared with MIME type "…" in content, but has MIME type "…" in the package document.
`OPF-014`	Error	The property "…" should be declared in the OPF file.
`OPF-015`	Error	The property "…" should not be declared in the OPF file.
`OPF-016`	Error	The element "rootfile" is missing its required attribute "full-path".
`OPF-017`	Error	The attribute "full-path" on element "rootfile" must not be empty.
`OPF-018`	Warning	The "remote-resources" property was declared in the Package Document, but no reference to remote resources has been found.
`OPF-018b`	Usage	The "remote-resources" property was declared in the Package Document, but no reference to remote resources has been found; please check scripted content to make sure the property is legit.
`OPF-021`	Warning	Use of non-registered URI scheme type in href: "…".
`OPF-025`	Error	Property value list "…" is not allowed, only one value must be specified.
`OPF-026`	Error	Found malformed property value: "…".
`OPF-027`	Error	Undefined property: "…".
`OPF-028`	Error	Undeclared prefix: "…".
`OPF-029`	Error	The file "…" does not appear to match the media type …, as specified in the OPF file.
`OPF-030`	Error	The unique-identifier "…" was not found.
`OPF-031`	Error	File listed in reference element in guide was not declared in OPF manifest: ….
`OPF-032`	Error	Guide references "…" which is not a valid "OPS Content Document".
`OPF-033`	Error	The spine contains no linear resources.
`OPF-034`	Error	The spine contains multiple references to the manifest item with id "…".
`OPF-035`	Warning	Media type "text/html" is not appropriate for XHTML/OPS.
`OPF-036`	Usage	Video type "…" might not be supported by reading systems
`OPF-037`	Warning	Found deprecated media-type "…".
`OPF-038`	Warning	Media type "…" is not appropriate for an OEBPS 1.2 context; Use "text/x-oeb1-document" instead.
`OPF-039`	Warning	Media-type "…" is not appropriate in an OEBPS 1.2 context. Use "text/x-oeb1-css" instead.
`OPF-040`	Error	Fallback item with id "…" could not be found.
`OPF-041`	Error	Fallback-style item with id "…" could not be found.
`OPF-042`	Error	"…" is not a permissible spine media-type.
`OPF-043`	Error	Spine item with non-standard media-type "…" has no fallback.
`OPF-044`	Error	Spine item with non-standard media-type "…" has no EPUB content document fallback.
`OPF-045`	Error	Encountered circular reference in fallback chain.
`OPF-047`	Usage	OPF file is using OEBPS 1.2 syntax allowing backwards compatibility.
`OPF-048`	Error	Package tag is missing its required unique-identifier attribute and value.
`OPF-049`	Error	Item id "…" was not found in the manifest.
`OPF-050`	Error	TOC attribute references resource with non-NCX mime type; "application/x-dtbncx+xml" is expected.
`OPF-051`	Suppressed	Image dimensions exceed recommended size.
`OPF-052`	Error	Role value "…" is not valid.
`OPF-053`	Warning	Date value "…" does not follow recommended syntax as per http://www.w3.org/TR/NOTE-datetime:….
`OPF-054`	Error	Date value "…" is not valid as per http://www.w3.org/TR/NOTE-datetime:….
`OPF-055`	Warning	… tag is empty.
`OPF-057`	Suppressed	Image file length exceeds recommended size.
`OPF-060`	Error	Duplicate entry in the ZIP file: "…" (file names must be unique after Unicode canonical normalization and full case folding).
`OPF-062`	Usage	Found Adobe page-map attribute on spine element in opf file.
`OPF-063`	Warning	Referenced Adobe page-map item "…" was not found in the manifest.
`OPF-064`	Info	OPF declares type "…", validating using profile "…".
`OPF-065`	Error	Invalid metadata declaration, probably due to a cycle in "refines" metadata.
`OPF-066`	Error	Missing "dc:source" or "source-of" pagination metadata. The pagination source must be identified using the "dc:source" and "source-of" properties when the content includes page break markers.
`OPF-067`	Error	The resource "…" must not be listed both as a "link" element in the package metadata and as a manifest item.
`OPF-070`	Warning	Custom collection role "…" is an invalid URL.
`OPF-071`	Error	Index collections must only contain resources pointing to XHTML Content Documents.
`OPF-072`	Usage	Metadata element "…" is empty.
`OPF-073`	Error	External identifiers must not appear in the document type declaration.
`OPF-074`	Error	Package resource "…" is declared in several manifest items.
`OPF-075`	Error	Preview collections must only point to EPUB Content Documents.
`OPF-076`	Error	The URI of preview collections link elements must not include EPUB canonical fragment identifiers.
`OPF-077`	Warning	A Data Navigation Document should not be included in the spine.
`OPF-078`	Error	An EPUB Dictionary must contain at least one Content Document with dictionary content (epub:type "dictionary").
`OPF-079`	Warning	Dictionary content was found (epub:type "dictionary"), the Package Document should declare the dc:type "dictionary".
`OPF-080`	Warning	A Search Key Map document file name should have the extension ".xml".
`OPF-081`	Error	Resource "…" (referenced from an EPUB Dictionary collection) was not found.
`OPF-082`	Error	Found an EPUB Dictionary collection containing more than one Search Key Map Document.
`OPF-083`	Error	Found an EPUB Dictionary collection containing no Search Key Map Document.
`OPF-084`	Error	Found an EPUB Dictionary collection containing resource "…" which is neither a Search Key Map Document nor an XHTML Content Document.
`OPF-085`	Warning	"dc:identifier" value "…" is marked as a UUID, but is an invalid UUID.
`OPF-086`	Warning	Property "…" is deprecated. Consider using … instead.
`OPF-086b`	Usage	epub:type value "…" is deprecated. Consider using … instead.
`OPF-086c`	Warning	the "…" reserved prefix is deprecated.
`OPF-087`	Usage	epub:type value "…" is not allowed on documents of type "…".
`OPF-088`	Usage	Unrecognized epub:type value "…".
`OPF-089`	Error	The "alternate" link rel keyword cannot be paired with other keywords.
`OPF-090`	Usage	It is encouraged to use MIME media type "…" instead of "…".
`OPF-091`	Error	The item href URL must not have a fragment identifier.
`OPF-092`	Error	Language tag "…" is not well-formed: …
`OPF-093`	Error	The "media-type" attribute is required for linked resources located in the EPUB container
`OPF-094`	Error	The "media-type" attribute is required for "…" links.
`OPF-095`	Error	The "media-type" attribute of "voicing" links must be an audio MIME type, but found "…".
`OPF-096`	Error	Non-linear content must be reachable, but found no hyperlink to "…".
`OPF-096b`	Usage	No hyperlink was found to non-linear document "…", please check that it can be reached from scripted content.
`OPF-097`	Usage	Resource "…" is listed in the manifest, but no reference to it was found in content documents.
`OPF-098`	Error	The "href" attribute must reference resources, not elements in the package document, but found URL "…".
`OPF-099`	Error	The manifest must not list the package document.
`OPF-100`	Usage	The "…" property only applies to pre-paginated content.

HTM: XHTML content documents

The XHTML chapters: DOCTYPE, entities, namespaces and fixed-layout viewports.

Code	Default severity	Message
`HTM-001`	Error	Any publication resource that is an XML-based media type must be a valid XML 1.0 document. XML version found: ….
`HTM-002`	Warning	The installed xml parser doesn’t support xml version verification. Xml files must be a valid XML 1.0 document.
`HTM-003`	Error	External entities are not allowed in EPUB v3 documents. External entity declaration found: ….
`HTM-004`	Error	Irregular DOCTYPE: found "…", expected "…".
`HTM-005`	Usage	An external reference was found.
`HTM-007`	Warning	Empty or whitespace-only value of attribute ssml:ph.
`HTM-009`	Error	The DOCTYPE provided is obsolete or irregular and can be removed.
`HTM-010`	Usage	Namespace "…" is unusual for the prefix "epub".
`HTM-011`	Error	Entity is undeclared.
`HTM-025`	Warning	Non-registered URI scheme type found in href.
`HTM-044`	Usage	Namespace uri "…" was included but not used.
`HTM-045`	Usage	Encountered empty href.
`HTM-046`	Error	Fixed layout document has no "viewport" meta element.
`HTM-047`	Error	Viewport metadata "…" has a syntax error
`HTM-048`	Error	SVG Fixed-Layout Documents must have a "viewBox" attribute (on the outermost "svg" element).
`HTM-051`	Warning	Found Microdata semantic enrichments but no RDFa. EDUPUB recommends using RDFa Lite.
`HTM-052`	Error	The property "region-based" is only allowed on nav elements in Data Navigation Documents.
`HTM_054`	Error	Custom attribute namespace ("…") must not include the string "…" in its domain.
`HTM_055`	Usage	The "…" element should not be used (discouraged construct)
`HTM_056`	Error	Viewport metadata has no "…" dimension (both "width" and "height" properties are required)
`HTM_057`	Error	Viewport "…" value must be a positive number or the keyword "device-…"
`HTM_058`	Error	HTML documents must be encoded in UTF-8, but UTF-16 was detected.
`HTM_059`	Error	Viewport "…" property must not be defined more than once, but found values […].
`HTM_060a`	Usage	EPUB reading systems must ignore secondary viewport meta elements in fixed-layout documents; viewport declaration "…" will be ignored.
`HTM_060b`	Usage	EPUB reading systems must ignore viewport meta elements in reflowable documents; viewport declaration "…" will be ignored.
`HTM_061`	Error	"…" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).
`HTM_062`	Usage	SVG "xlink:href" attribute is deprecated; an "href" attribute should be used instead.
`HTM_063`	Usage	SVG "xlink:href" attribute value ("…") should be equal to the "href" attribute value ("…").

CSS: Stylesheets

CSS files and style elements: forbidden properties, encodings, fonts and syntax.

Code	Default severity	Message
`CSS-001`	Error	The "…" property must not be included in an EPUB Style Sheet.
`CSS-002`	Error	Empty or NULL reference found.
`CSS-003`	Warning	CSS document is encoded in UTF-16. It should be encoded in UTF-8 instead.
`CSS-004`	Error	CSS documents must be encoded in UTF-8, detected …;
`CSS-005`	Usage	Conflicting alternate style tags found: ….
`CSS-006`	Usage	CSS selector specifies fixed position.
`CSS-007`	Info	Font-face reference "…" refers to non-standard font type "…".
`CSS-008`	Error	An error occurred while parsing the CSS: ….
`CSS-015`	Error	Alternative style sheets must have a title.
`CSS-019`	Warning	CSS font-face declaration has no attributes.
`CSS-028`	Usage	Use of Font-face declaration.
`CSS-029`	Usage	Found CSS class name "…" but no "…" property was declared in the package document.
`CSS-030`	Error	The package document declares media overlays styling class names but no CSS was found in the content document.

NAV: Navigation document

The EPUB 3 table of contents, page list and landmarks.

Code	Default severity	Message
`NAV-001`	Error	The nav file is not supported for EPUB v2.
`NAV-003`	Error	The Navigation Document must have a page list when content document(s) contain page breaks (epub:type="pagebreak").
`NAV-004`	Usage	The Navigation Document should contain the full document heading hierarchy in EDUPUB.
`NAV-005`	Usage	Content documents contain "audio" elements but the Navigation Document does not have a listing of audio clips (epub:type="loa").
`NAV-006`	Usage	Content documents contain "figure" elements but the Navigation Document does not have a listing of figures (epub:type="loi").
`NAV-007`	Usage	Content documents contain "table" elements but the Navigation Document does not have a listing of tables (epub:type="lot").
`NAV-008`	Usage	Content documents contain "video" elements but the Navigation Document does not have a listing of video clips (epub:type="lov").
`NAV-009`	Error	Region-based navigation links must point to Fixed-Layout Documents.
`NAV-010`	Error	"…" nav must not link to remote resources; found link to "…".
`NAV-011`	Warning	"…" nav must be in reading order; link target "…" is before the previous link’s target in … order.

NCX: NCX (EPUB 2 table of contents)

The legacy NCX table of contents.

Code	Default severity	Message
`NCX-001`	Error	NCX identifier ("…") does not match OPF identifier ("…").
`NCX-004`	Usage	NCX identifier ("dtb:uid" metadata) should not contain leading or trailing whitespace.
`NCX-006`	Usage	Empty "text" label in the NCX document

MED: Images, audio and media overlays

Image formats and headers, and synchronised audio (media overlays).

Code	Default severity	Message
`MED-003`	Error	Picture "img" elements must reference core media type resources, but found resource "…" of type "…".
`MED-004`	Error	Image file header may be corrupted.
`MED-005`	Error	Media Overlay audio reference … to non-standard audio type … found.
`MED-008`	Error	The time specified in the clipBegin attribute must not be after clipEnd.
`MED-009`	Error	The time specified in the clipBegin attribute must not be the same as clipEnd.
`MED_007`	Error	Picture "source" elements must define a "type" attribute when they reference foreign resources; found resource "…" of type "…".
`MED_010`	Error	EPUB Content Documents referenced from a Media Overlay must specify the "media-overlay" attribute.
`MED_011`	Error	EPUB Content Document referenced from multiple Media Overlay Documents.
`MED_012`	Error	The "media-overlay" attribute does not match the ID of the Media Overlay that refers to this document.
`MED_013`	Error	Media Overlay Document referenced from the "media-overlay" attribute does not contain a reference to this Content Document.
`MED_014`	Error	Media overlay audio file URLs must not have a fragment, but found '#…'.
`MED_015`	Usage	Media overlay text references must be in reading order. Text target "…" is before the previous link target in … order.
`MED_016`	Warning	Media Overlays total duration should be the sum of the durations of all Media Overlays documents.
`MED_017`	Warning	URL fragment should indicate an element ID, but found '#…'.
`MED_018`	Warning	URL fragment should be an SVG fragment identifier, but found '#…'.

ACC: Accessibility

Accessibility checks on content documents (most are suppressed by default).

Code	Default severity	Message
`ACC-004`	Suppressed	Html "a" element must have text.
`ACC-005`	Suppressed	Table heading cells should be identified by "th" elements for accessibility.
`ACC-006`	Suppressed	Tables should include a "thead" element for accessibility.
`ACC-007`	Suppressed	Content Documents do not use "epub:type" attributes for semantic inflection.
`ACC-009`	Usage	MathML should either have an "alttext" attribute or "annotation-xml" child element.
`ACC-011`	Usage	SVG hyperlink has no accessible name
`ACC-012`	Suppressed	Table elements should include a caption element.

OBS: Obsolete features

Features that still work but are outdated.

Code	Default severity	Message
`OBS-001`	Usage	Usage of … is outdated.

INF: Information

Notes about EPUBCheck itself.

Code	Default severity	Message
`INF-001`	Info	Rule … is under review and its severity may change in a future release. See the discussion at …

CHK: EPUBCheck configuration

Problems with EPUBCheck's own custom message file, not with the EPUB.

Code	Default severity	Message
`CHK-001`	Error	The custom message overrides file was not found.
`CHK-002`	Error	Unrecognized custom message id … encountered in message overrides file "…".
`CHK-003`	Error	Unrecognized custom message severity "…" encountered in message overrides file "…".
`CHK-004`	Error	The custom message contains too many parameters in message overrides file "…".
`CHK-005`	Error	The custom suggestion contains too many parameters in message overrides file "…".
`CHK-006`	Error	Unable to parse the custom format parameter in message overrides file "…".
`CHK-007`	Error	Error encountered while processing custom message file "…": "…".
`CHK-008`	Error	Error encountered while processing an item "…"; skip other checks for the item.

Retired message IDs

These IDs still exist in EPUBCheck's list of messages but have no text and are suppressed by default, so EPUBCheck never reports them unless a custom message file turns them on.

ACC-001, ACC-002, ACC-003, ACC-008, ACC-010, ACC-013, ACC-014, ACC-015, ACC-016, ACC-017, CSS-009, CSS-010, CSS-011, CSS-012, CSS-013, CSS-016, CSS-017, CSS-020, CSS-021, CSS-022, CSS-023, CSS-024, CSS-025, HTM-006, HTM-008, HTM-012, HTM-013, HTM-014, HTM-014a, HTM-015, HTM-016, HTM-017, HTM-018, HTM-019, HTM-020, HTM-021, HTM-022, HTM-023, HTM-024, HTM-027, HTM-028, HTM-029, HTM-033, HTM-036, HTM-038, HTM-049, HTM-050, HTM_053, MED-001, MED-002, MED_006, NAV-002, NCX-002, NCX-003, NCX-005, OPF-008, OPF-009, OPF-019, OPF-020, OPF-046, OPF-056, OPF-058, OPF-059, OPF-068, OPF-069, RSC-018, RSC-023, SCP-001, SCP-002, SCP-003, SCP-004, SCP-005, SCP-006, SCP-007, SCP-008, SCP-009, SCP-010

Source and licence

Message texts and default severities come from EPUBCheck 5.4.0, the W3C's EPUB validator, released under the BSD-3-Clause licence. Copyright © 2007 Adobe Systems Incorporated. Copyright © 2008 IDPF. Copyright © 2017 W3C (MIT, ERCIM, Keio, Beihang). github.com/w3c/epubcheck

Causes and fixes are written by EpubCC. Codes without an explanation show only the official message and are not offered to search engines until we write one. EpubCC is not affiliated with the W3C.