ISO/IEC 23736-6:2020

INTERNATIONAL ISO/IEC
STANDARD 23736-6
First edition
2020-02
Information technology — Digital
publishing — EPUB 3.0.1 —
Part 6:
Canonical fragment identifiers
Technologies de l'information — Publications numériques — EPUB
3.0.1 —
Partie 6: Identificateurs de fragment canoniques
Reference number
ISO/IEC 23736-6:2020(E)
©
ISO/IEC 2020

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ISO/IEC 23736-6:2020(E)

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Published in Switzerland
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ISO/IEC 23736-6:2020(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
committees established by the respective organization to deal with particular fields of technical activity.
ISO and IEC technical committees collaborate in fields of mutual interest. Other international
organizations, governmental and non‐governmental, in liaison with ISO and IEC, also take part in the
work.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of document should be noted (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC
list of patent declarations received (see http://patents.iec.ch).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT),
see www.iso.org/iso/foreword.html.
This document was prepared by the World Wide Web Consortium (W3C) (as EPUB Canonical Fragment
Identifiers 1.1) and drafted in accordance with its editorial rules. It was adopted, under the JTC 1
PAS procedure, by Joint Technical Committee ISO/IEC JTC 1, Information technology.
A list of all parts in the ISO/IEC 23736 series can be found on the ISO websitte.e
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO/IEC 23736-6:2020(E)
EPUB Canonical Fragment Identifiers 1.1
Recommended Specification 5 January 2017
This version
http://www.idpf.org/epub/linking/cfi/epub-cfi-20170105.html
Latest version
http://www.idpf.org/epub/linking/cfi/epub-cfi.html
Previous version
http://www.idpf.org/epub/linking/cfi/epub-cfi-20161130.html
Previous recommendation
http://www.idpf.org/epub/linking/cfi/epub-cfi-20140628.html
Document history
Changes to this document
Issues addressed in this revision
Report an issue
Errata
Editors
Peter Sorotokin, Adobe
Garth Conboy, Google Inc.
Brady Duga, Google Inc.
John Rivlin, Google Inc.
Don Beaver, Apple Inc.
Kevin Ballard, Apple Inc.
Alastair Fettes, Apple Inc.
Daniel Weck, DAISY Consortium
Copyright © 2011 International Digital Publishing Forum™
All rights reserved. This work is protected under Title 17 of the United States Code. Reproduction and
dissemination of this work with changes is prohibited except with the written permission of the International
Digital Publishing Forum (IDPF).
EPUB is a registered trademark of the International Digital Publishing Forum.
Status of this Document
This section describes the status of this document at the time of its publication. Other
documents might supersede this document.
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ISO/IEC 23736-6:2020(E)
This document was produced by the EPUB Working Group under the EPUB Working
Group Charter approved on 8 July 2015.
This document has been reviewed by the IDPF membership and is endorsed by the IDPF
Board as a Recommended Specification. This document is considered stable and can be
referenced from other specifications and documents.
Feedback on this document can be provided to the EPUB Working Group's mailing list or
issue tracker.
This document is governed by the IDPF Policies and Procedures.
Table of Contents
1. Overview
1.1. Purpose and Scope
1.2. Terminology
1.3. Typographic Conventions
1.4. Conformance Statements
2. EPUB CFI Definition
2.1. Introduction
2.2. Syntax
2.3. Character Escaping
3. EPUB CFI Processing
3.1. Path Resolution
3.1.1. Step Reference to Child Element or Character Data (/)
3.1.2. XML ID Assertion ([)
3.1.3. Step Indirection (!)
3.1.4. Character Offset (:)
3.1.5. Temporal Offset (~)
3.1.6. Spatial Offset (@)
3.1.7. Temporal-Spatial Offset (~ + @)
3.1.8. Text Location Assertion ([)
3.1.9. Side Bias ([ + ;s=)
3.1.10. Examples
3.2. Sorting Rules
3.3. Intra-Publication CFIs
3.4. Simple Ranges
3.5. Intended Target Location Correction
4. Extending EPUB CFIs
References
› 1 Overview
› 1.1 Purpose and Scope
This section is informative
This specification, EPUB Canonical Fragment Identifier (epubcfi), defines a standardized
method for referencing arbitrary content within an EPUB® Publication through the use of
fragment identifiers.
The Web has proven that the concept of hyperlinking is tremendously powerful, but EPUB
Publications have been denied much of the benefit that hyperlinking makes possible
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ISO/IEC 23736-6:2020(E)
because of the lack of a standardized scheme to link into them. Although proprietary
schemes have been developed and implemented for individual Reading Systems, without
a commonly-understood syntax there has been no way to achieve cross-platform
interoperability. The functionality that can see significant benefit from breaking down this
barrier, however, is varied: from reading location maintenance to annotation attachment to
navigation, the ability to point into any Publication opens a whole new dimension not
previously available to developers and Authors.
This specification attempts to rectify this situation by defining an arbitrary structural
reference that can uniquely identify any location, or simple range of locations, in an EPUB
Publication: the EPUB CFI. The following considerations have strongly influenced the
design and scope of this scheme:
The mechanism used to reference content should be interoperable: references to a
reading position created by one Reading System should be usable by another.
Document references to EPUB content should be enabled in the same way that
existing hyperlinks enable references throughout the Web.
Each location in an EPUB file should be able to be identified without the need to
modify the document.
All fragment identifiers that reference the same logical location should be equal
when compared.
Comparison operations, including tests for sorting and comparison, should be able
to be performed without accessing the referenced files.
Simple manipulations should be possible without access to the original files (e.g.,
given a reference deep in a file, it should be possible to generate a reference to the
start of the file).
Identifier resolution should be reasonably efficient (e.g., processing of the first
chapter is not necessary to resolve a fragment identifier that points to the last
chapter).
References should be able to recover their target locations through parser variations
and document revisions.
Expression of simple, contiguous ranges should be supported.
An extensible mechanism to accommodate future reference recovery heuristics
should be provided.
In the case of both Standard EPUB CFIs and Intra-Publication EPUB CFI, this
specification conforms with the guidelines expressed by W3C in Section 6. Best Practices
for Fragid Structures [FragIDBestPractices].
In other words, both standard CFI URIs (e.g., "book.epub#epubcfi(…)", referred media type
"application/epub+zip") and intra-publication CFI URIs (e.g., "package.opf#epubcfi(…)",
referred media type "application/oebps-package+xml") make use of a fragment identifier
syntax that does not overlap with existing schemes in the context of the aforementioned
media types' suffix registrations (i.e., "-xml" and "-zip").
› 1.2 Terminology
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ISO/IEC 23736-6:2020(E)
Please refer to [EPUB 3.1] for definitions of EPUB-specific terminology used in this
document.
Standard EPUB CFI
A publication-level EPUB CFI links into an EPUB Publication. The path
preceding the EPUB CFI references the location of the EPUB Publication.
Intra-Publication EPUB CFI
An intra-publication EPUB CFI allows one Content Document to reference
another within the same Rendition of an EPUB Publication. The path preceding
the EPUB CFI references the current Rendition's Package Document.
Refer to Intra-Publication CFIs for more information.
› 1.3 Typographic Conventions
The following typographic conventions are used in this specification:
markup
All markup (elements, attributes, properties), code (JavaScript, pseudo-code),
machine-readable values (string, characters, media types) and file names are in red
monospace font.
markup link
Links to markup and code definitions are in underlined red monospace font.
http://www.idpf.org/
URIs are in navy blue monospace font.
hyperlink
Hyperlinks are underlined and blue.
[reference]
Normative and informative references are enclosed in square brackets.
Term
Terms defined in the Terminology are in capital case.
Term Link
Links to term definitions have a dotted blue underline.
Normative element, attribute and property definitions are in blue boxes.
Informative markup examples are in light gray boxes.
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ISO/IEC 23736-6:2020(E)
NOTE
Informative notes are in green boxes with a "Note" header.
CAUTION
Informative cautionary notes are in red boxes with a "Caution" header.
› 1.4 Conformance Statements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT,
RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in
[RFC2119].
All sections and appendixes of this specification are normative except where identified by
the informative status label "This section is informative". The application of informative
status to sections and appendixes applies to all child content and subsections they
contain.
All examples in this specification are informative.
› 2 EPUB CFI Definition
› 2.1 Introduction
This section is informative
A fragment identifier is the part of an IRI [RFC3987] that defines a location within a
resource. Syntactically, it is the segment attached to the end of the resource IRI starting
with a hash (#). For HTML documents, IDs and named anchors are used as fragment
identifiers, while for XML documents the Shorthand XPointer [XPTRSH] notation is used
to refer to a given ID.
A Canonical Fragment Identifier (CFI) is a similar construct to these, but expresses a
location within an EPUB Publication. For example:
book.epub#epubcfi(/6/4[chap01ref]!/4[body01]/10[para05]/3:10)
The function-like string immediately following the hash (epubcfi(…)) indicates that this
fragment identifier conforms to the scheme defined by this specification, and the value
contained in the parentheses is the syntax used to reference the location within the
specified EPUB Publication (book.epub). Using the processing rules defined in Path
Resolution, any Reading System can parse this syntax, open the corresponding Content
Document in the EPUB Publication and load the specified location for the user.
A complete definition of the EPUB CFI syntax is provided in the next section.
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ISO/IEC 23736-6:2020(E)
NOTE
epub has been prepended to the name of the scheme, as a more generic CFI-like
scheme might be defined in the future for all XML+ZIP-based file formats.
› 2.2 Syntax
(EBNF productions ISO/IEC 14977)
All terminal symbols are in the Unicode Block 'Basic Latin' (U+0000 to U+007F).
fragment = "epubcfi(" , ( path , [ range ] ) , ")" ;
path = step , local_path ;
range = "," , local_path , "," , local_path ;
local_path = { step } , ( redirected_path | [ offset ] );
redirected_path = "!" , ( offset | path );
step = "/" , integer , [ "[" , assertion , "]" ] ;
offset = ( ( ":" , integer ) | ( "@" , number , ":" , number ) | ( "~" , number
, [ "@" , number , ":" , number ] ) ) , [ "[" , assertion , "]" ] ;
number = ( digit-non-zero , { digit } , [ "." , { digit } , digit-non-zero ] ) | (
zero , [ "." , { digit } , digit-non-zero ] ) ;
integer = zero | ( digit-non-zero , { digit } ) ;
assertion = ( ( value , [ "," , value ] ) | ( "," , value ) | ( parameter ) ) {
parameter } ;
parameter = ";" , value-no-space , "=" , csv ;
csv = value , { "," , value } ;
value = string-escaped-special-chars ;
value-no-space = value - ( [ value ] , space , [ value ] ) ;
special-chars = circumflex | square-brackets | parentheses | comma |
semicolon | equal ;
escaped- = ( circumflex , circumflex ) | ( circumflex , square-brackets ) | (
special-chars circumflex , parentheses ) | ( circumflex , comma ) | (
circumflex , semicolon ) | ( circumflex , equal ) ;
character- = ( character - special-chars ) | escaped-special-chars ;
escaped-
special
string-escaped- = character-escaped-special , { character-escaped-special } ;
special-chars
digit = zero | digit-non-zero ;
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ISO/IEC 23736-6:2020(E)
digit-non-zero = "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
zero = "0" ;
space = " " ;
circumflex = "^" ;
square- = "[" | "]" ;
brackets
parentheses = "(" | ")" ;
comma = "," ;
semicolon = ";" ;
equal = "=" ;
character = ? Unicode Characters ? ;
› Unicode Characters
The definition of allowed Unicode characters is the same as [XML]. This excludes the
surrogate blocks, FFFE, and FFFF:

#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
Document authors are encouraged to avoid "compatibility characters", as defined in
section 2.3 of [Unicode]. The characters defined in the following ranges are also
discouraged. They are either control characters or permanently undefined Unicode
characters:

[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDEF],
[#x1FFFE-#x1FFFF], [#x2FFFE-#x2FFFF], [#x3FFFE-#x3FFFF],
[#x4FFFE-#x4FFFF], [#x5FFFE-#x5FFFF], [#x6FFFE-#x6FFFF],
[#x7FFFE-#x7FFFF], [#x8FFFE-#x8FFFF], [#x9FFFE-#x9FFFF],
[#xAFFFE-#xAFFFF], [#xBFFFE-#xBFFFF], [#xCFFFE-#xCFFFF],
[#xDFFFE-#xDFFFF], [#xEFFFE-#xEFFFF], [#xFFFFE-#xFFFFF],
[#x10FFFE-#x10FFFF].
A Canonical Fragment Identifier (CFI) consists of an initial sequence epubcfi that
identifies this particular reference method, and a parenthesized path or range. A path is
built up as a sequence of structural steps to reference a location. A range is a path
followed by two local (or relative) paths that identify the start and end of the range.
Steps are denoted by the forward slash character (/), and are used to traverse XML
content. The last step in a CFI path represents a location within a document, either
structural (XML element), textual (character data), or aural-visual (image, audio, or video
media). Such terminating steps MAY be complemented by an OPTIONAL "offset", which
denotes a particular character position, temporal or spatial fragment.
Substrings in brackets are extensible assertions that improve the robustness of traversing
paths and migrating them from one revision of the document to another. These assertions
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ISO/IEC 23736-6:2020(E)
preserve additional information about traversed elements of the document, which makes it
possible to recover intended location even after some modifications are made to the
EPUB Publication.
Although the value definition in the syntax above allows any a sequence of characters, a
circumflex (^) MUST be used to escape the following characters to ensure their presence
does not interfere with parsing:
brackets ([,])
circumflex (^)
comma (,)
parentheses ((,))
semicolon (;)
Example of an EPUB CFI that points to a location after the text "2[1]".
epubcfi(/6/14[chap05ref]!/4[body01]/10/2/1:3[2^[1^]])
The following rules apply to the use of numbers and integers within the path or range:
leading zeros are not allowed for numbers or integers (to ensure uniqueness);
trailing zeros are not allowed in the fractional part of a number;
zero MUST be represented as the integer 0;
numbers in the range 1 > N > 0 MUST have a leading 0.;
integral numbers MUST be represented as integers.
› 2.3 Character Escaping
As described in Syntax, the EPUB CFI grammar contains characters that have a special
purpose as delimiters within a fragment identifier expression. These characters MUST be
escaped using the circumflex '^' character when not intended for use as delimiters, so that
they can appear within the EPUB CFI data without being mistaken for delimiters.
Depending on the usage context of such EPUB CFI, further character escaping MAY be
necessary in order to ensure that all potentially-conflicting text tokens are encoded
correctly.
IRI and URI references:
The EPUB CFI (fragment identifier) scheme is designed to be used within URI
and IRI references. The [RFC3986] specification defines a number of
"reserved" characters that have a specific purpose as delimiters, and which
MAY need to be escaped in cases when they would otherwise conflict with the
syntactical structure of the URI/IRI reference. The character used for escaping
is the percent sign '%', and escapable characters get percent-encoded. For
example, the percent character itself becomes "%25" when it gets escaped
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ISO/IEC 23736-6:2020(E)
(note the difference with EPUB CFI's circumflex '^', which gets escaped using
a double character '^^').
Unlike IRI references, URI references require unicode characters to be ASCII-
encoded. Although the EPUB specification itself is based on IRIs (i.e. authors
and production tools are expected to use IRIs), some systems or APIs might
only support URIs. As a result, implementors MAY still need too handle the
conversion of IRI to URI references, as defined in [RFC3987]. Disallowed
characters are escaped as follows:
Each disallowed character is converted to UTF-8 [RFC2279] as one or
more bytes. The disallowed characters in URI references include all non-
ASCII characters, plus the excluded characters listed in Section 2.4 of
[RFC2396], except for the number sign '#' and percent sign '%' and the
square bracket characters re-allowed in [RFC2732].
The resulting bytes are escaped with the URI escaping mechanism (that
is, converted to '%HH', where HH is the hexadecimal notation of the byte
value).
The original character is replaced by the resulting character sequence.
(X)HTML context:
IRI references are designed to be used in the various types of documents that EPUB
Publications comprise. XML and (X)HTML represent yet another insertion context
that requires specific character escaping rules. For example, double quote
characters or angle brackets conflict with significant delimiters in the markup syntax,
and MUST therefore be escaped using the &xxx; special sequence (character
reference).
When multiple layers of character escaping are applied to escape or unescape an EPUB
CFI, they MUST be applied in reverse order to revert back to the original form. For example,
[ EPUB-CFI -> IRI -> (X)HTML ] becomes [ (X)HTML -> IRI -> EPUB-CFI ]
The following example shows an EPUB CFI in its "raw" form (only with '^' circumflex
escaping). Note the assertion text at the end of it, with escaped square brackets as well as the
escaped circumflex character itself (the unescaped text is 'Ф-"spa ce"-99%-aa[bb]^'):
epubcfi(/6/4!/4/10/2/1:3[Ф-"spa ce"-99%-aa^[bb^]^^])
When taking part in an IRI, the space character within the assertion might become percent-
escaped ('%20'), and the percent character itself MUST be escaped ('%25'). Note that the
square brackets '[' ']' and semicolumn ':' are "reserved" characters (as per the URI
specification) but because they serve no purpose as delimiters when the IRI processor
extracts the fragment identifier, they do not need to be escaped (i.e. the fragment component
of the IRI can non-ambiguously be parsed by processing all the text after the '#' character).
The circumflex '^' also falls within the category of "unwise" (or "unsafe") characters, but the
EPUB fragment identifier scheme does not require escaping them. Here is the IRI-escaped
EPUB CFI:
#epubcfi(/6/4!/4/10/2/1:3[Ф-"spa%20ce"-99%25-aa^[bb^]^^])
When the IRI appears within an XML attribute, the double quote character (quotation mark) is
significant as a delimiter of the attribute value, so it becomes escaped with '"'. Note that
the Cyrillic "EF" character ('Ф') is directly supported in EPUB XML documents (which use the
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ISO/IEC 23736-6:2020(E)
UTF-8 encoding to represent the unicode character repertoire), so it doesn't need to be
encoded:
#epubcfi(/6/4!/4/10/2/1:3[Ф-"spa%20ce"-99%25-aa^[bb^]^^])
If the IRI need to be converted to URI, the non-ASCII Cyrillic "EF" character ('Ф') would get
percent-escaped with 2 bytes ('0xd0 0xa4', in hexadecimal). This would result in the
following URI:
#epubcfi(/6/4!/4/10/2/1:3[%d0%a4-%22spa%20ce%22-99%25-aa^[bb^]^^])
URI encoding / decoding APIs usually "aggressively" percent-encode characters, as
demonstrated in the following example. Note how the circumflexes '^' (%5E), square brackets
'[' (%5B) ']' (%5D) and double-quotes '"' (%22) are also percent-encoded (due to their
"unsafe" / "unwise" nature within URIs) :
#epubcfi(/6/4!/4/10/2/1:3%5B%D0%A4-%22spa%20ce%22-99%25-
aa%5E%5Bbb%5E%5D%5E%5E%5D)
› 3 EPUB CFI Processing
› 3.1 Path Resolution
The process of resolving an EPUB CFI to a location within an EPUB Publication begins
with the root package element of the Package Document. Each step in the CFI is then
processed one by one, left to right, applying the rules defined in the following subsections.
NOTE
The EPUB CFI examples in the following subsections are based on the sample
documents in Examples.
› 3.1.1 Step Reference to Child Element or Character Data (/)
A step with a slash (/) followed by a positive integer refers to either a child element or a
chunk of character data, as per the rules defined herein:
[XML] content other than element and character data is ignored. Note that as per the
[XML] specification, character data inside CDATA sections is included, and
conversely, XML comments are ignored.
[XML] character data that corresponds to insignificant white space (typically used for
markup formatting/indenting) is preserved. Character and entity references are
considered expanded, and character data is obtained from the "included
replacement text" (as per the terminology defined in the [XML] specification).
[XML] character data that is interspersed amongst sibling child elements (i.e., "mixed
content" context) is logically organised into (potentially-empty) chunks of contiguous
character data: the first chunk is located before the first child element (left sibling),
the last chunk is located after the last child element (right sibling), and there is one
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ISO/IEC 23736-6:2020(E)
chunk between each pair of child elements. When there are no child elements, there
is one (potentially-empty) chunk of character data. Consecutive (potentially-empty)
chunks of character data are each assigned odd indices (i.e., starting at 1, followed
by 3, etc.).
Child [XML] elements are assigned even indices (i.e., starting at 2, followed by 4,
etc.). Additionally, 0 is a valid index that refers to a non-existing element which
virtually precedes the first potentially-empty chunk of character data within the
parent element's content. Similarly, n+2 is a valid index that refers to a non-existing
element which virtually follows the last potentially-empty chunk of character data,
where n is the even index of the last child element, or 0 if there are no child
elements. CFI processors (e.g., Reading Systems) MUST be capable of consuming
(e.g., parsing and interpreting) CFI expressions containing references to the 0 and
n+2 "virtual" elements, even when the first (or last, respectively) chunk of character
data is empty. Conversely, the *production* of such CFI expressions is governed by
the following conformance requirement: if the first chunk of character data is empty,
a CFI expression SHOULD NOT be constructed using a reference to the "virtual"
element at index 0, instead the "real" first child element (at index 2) SHOULD be
referred to. Similarly, if the last chunk of character data is empty, a CFI expression
SHOULD NOT be constructed using a reference to the "virtual" element at index n+2,
instead the "real" last child element (at index n) SHOULD be referred to.
NOTE
The "virtual" first / last elements mechanism might facilitate interoperability with certain
instances of DOM Ranges, whereby non-existing elements are used to span across
textual content without relying on character offsets at the start/end boundaries.
For a Standard EPUB CFI, the leading step in the CFI MUST start with a slash (/) followed
by an even number that references the spine child element of the Package Document's
root package element. The Package Document traversed by the CFI MUST be the one
specified as the Default Rendition in the EPUB Publication's META-INF/container.xml file
(i.e., the Package Document referenced by the first rootfile element in container.xml).
For an Intra-Publication EPUB CFI, the first step MUST start with a slash followed by a
node number that references a position in Package Document starting from the root
package element.
› 3.1.2 XML ID Assertion ([)
When an EPUB CFI references an element that contains an ID [XML], the corresponding
path step MUST include that ID in square brackets (i.e., after the slash (/) and even number
that identifies the element).
Specification of identifiers adds robustness to the CFI scheme: a Reading System can
determine that the location referenced by the CFI is not the original intended location, and
can use the identifier to compute the set of steps that reach the desired destination in the
content (see Intended Target Location Correction). The cost of this added robustness is
that comparison (and sorting) of CFI strings can be performed only after logically stripping
all bracketed substrings (see Sorting Rules).
› 3.1.3 Step Indirection (!)
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If a step, or a sequence of steps, points to an element that references another document,
the exclamation mark (!) MUST be used whenever that step is immediately followed by an
expression that applies to the referenced document ("indirection"). The following
expression is then resolved from the root element of the referenced XML document, or
from the targeted XML fragment (when specified).
Only the following references are honored:
For itemref in the Package Document spine, the reference is defined by the href
attribute of the corresponding item element in the manifest (i.e., that the itemref's
idref attribute references).
For [HTML] iframes and embed elements, references are defined by the src attribute
For the [HTML] object element, the reference is defined by the data attribute
For [SVG] image and use elements, references are defined by the xlink:href
attribute
NOTE
This scheme does not take into account hyperlinks, only embedding references.
Consequently, it is illegal to follow links from the [HTML] (or [SVG]) a element.
› 3.1.4 Character Offset (:)
A path terminating with a leading colon (:) followed by an integer refers to a character
offset. The given character offset MAY apply to an element only if this element is the
[HTML] img element with an alt attribute containing the text to which the character offset
applies.
For XML character data, the offset is zero-based and always refers to a position between
characters, so 0 means before the first character and a number equal to the total UTF-16
length means after the last ch
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