Asciidoctor EPUB3 Documentation

Asciidoctor EPUB3 is a set of Asciidoctor extensions for converting AsciiDoc documents directly to the EPUB3 and KF8/MOBI e-book formats.


Asciidoctor EPUB3 is not merely a converter from AsciiDoc to EPUB3 and KF8/MOBI. Rather, it’s a tool for creating highly aesthetic, professional, easy-to-read e-books. Let’s face it, many of the technical e-books out there—​especially those produced from software documentation—​are hideous. Asciidoctor EPUB3 is on a mission to disrupt the status quo.

Figure 1. An excerpt from an e-book produced by Asciidoctor EPUB3 shown in Day, Night, and Sepia modes.

Project Mission

The Asciidoctor EPUB3 project aims to produce EPUB3 documents that meet the following objectives:

Fully Semantic

Produce deeply semantic XHTML5 documents, including use of the recommended epub:type attribute.

Exceptional Readability

Readers should be drawn into the text so that they read and absorb it. Maximize the readability of the text using carefully crafted styles that are focused on:

  • Custom, readable fonts with strong UTF-8 character support

  • Sufficient line spacing and margins

  • Modular font size scale

  • Subtle, pleasing colors with good contrast

  • A responsive design that scales well from small to large screens

  • Widowed and orphaned content avoided where possible

Complete and Accurate Metadata

Fully populate the EPUB3 package metadata using information in the AsciiDoc source document.

Consistent Rendering

Render consistently across a broad range of EPUB3 (and select EPUB2+) e-readers and respond to any size screen.

Polish, Polish, and More Polish

Add polish to the final product such as font-based icons and callout numbers.

We believe that the e-books produced by Asciidoctor EPUB3 are the very best output you can expect to find in digital publishing today. Of course, there’s always room for improvement, so we’ll continue to work with you to achieve and maintain this goal.

Notable Features

  • Direct AsciiDoc to EPUB3 conversion

  • Direct AsciiDoc to KF8/MOBI conversion

  • Highly-aesthetic and readable styles with optimized text legibility

  • Respects font settings (if supported by the e-reader) without altering headings, code or icons

  • EPUB3 metadata, manifest and spine (assembled by Gepub)

  • Document metadata (title, authors, subject, keywords, etc.)

  • Internal cross reference links

  • Syntax highlighting with Rouge, CodeRay or Pygments

  • Unicode callout numbers

  • Page breaks avoided in block content (so much as it’s supported by the e-reader)

  • Orphan section titles avoided (so much as it’s supported by the e-reader)

  • Table border settings honored

  • Support for SVG images in the content

  • Stem blocks via AsciiMath

Project Status

Asciidoctor EPUB3 is currently alpha software. Use accordingly. Although the bulk of AsciiDoc content is converted, there’s still work needed to fill in gaps where conversion is incomplete or unstyled.
Asciidoctor EPUB3 only produces variable layout (i.e., reflowable) EPUB3 documents since this layout is best suited for the types of documents typically written in AsciiDoc. We may explore the use of fixed layout documents in the future if the need arises.

Converter Workflow

Asciidoctor EPUB3 takes an aggregate AsciiDoc document and any assets it references as input and produces an EPUB3 publication archive (often described as a “website in a box”).

Asciidoctor EPUB3 can also produce KF8/MOBI files, the format required for viewing on Amazon Kindle. The conversion to KF8/MOBI is performed by generating a slightly modified EPUB3 publication, then passing it through the KindleGen application. The EPUB3 publication, which can be thought of as the “digital master”, is altered to adhere to certain Amazon Kindle requirements.

Structuring your Manuscript

An EPUB3 archive is composed of multiple files. The content of each “chapter” is typically stored in a dedicated XHTML file. Therefore, the Asciidoctor EPUB3 converter “chunks” the AsciiDoc source document into multiple XHTML files to add to the EPUB3 archive. Like other converters, Asciidoctor EPUB3 handles this chunking task by automatically slicing up the XHTML output at predetermined heading levels.

When the doctype attribute is set to book, each top-level section will become a separate e-book "chapter" file. This includes preface, bibliography, appendix, etc. This behavior can be configured via the epub-chapter-level document attribute.

Otherwise, the whole document is converted to a single ebook chapter.

You may specify custom chapter filenames by assigning IDs to sections:

= Chapter

Here’s an example showing the structure of a book:

= Book Title
Author Name
:doctype: book
:imagesdir: images
//...and so on

== Chapter One

Some interesting text here.

== Chapter Two

Even more exciting stuff.

In older Asciidoctor EPUB3 versions, there were strict rules on document organization: a 'spine' master document with chapter includes. This is no longer the case. If you followed the old rules, chances are your document will work with the newer Asciidoctor EPUB3 either as-is or after minor adjustments.


All that’s needed to use Asciidoctor EPUB3 is Ruby 2.3 or newer and a few Ruby gems (including at least Asciidoctor 1.5.6), which we’ll explain how to install in the next section.

To check if you have Ruby available, use the ruby command to query the installed version:

$ ruby --version

Getting Started

You can get Asciidoctor EPUB3 by installing the published gem.

Install the Published Gem

Asciidoctor EPUB3 is published on You can install the published gem using the following command:

$ NOKOGIRI_USE_SYSTEM_LIBRARIES=1 gem install asciidoctor-epub3

This optional environment variable tells the gem installer to link against the C libraries on the system, if available, instead of compiling the libraries from scratch. This speeds up the installation of Nokogiri considerably.

If you want to syntax highlight source listings, you’ll also want to install Rouge, CodeRay, or Pygments. Choose one (or more) of the following:

$ gem install rouge
$ gem install coderay
$ gem install pygments.rb

You then activate syntax highlighting for a given document by adding the source-highlighter attribute to the document header (Rouge shown):

:source-highlighter: rouge
If a style is not specified, the black and white theme (i.e., bw) is used. This default is used so that the syntax highlighting is legible regardless of which reading mode the reader selects (white, black, sepia, etc). To override this default, you must set <highlighter>-style document header attribute to a valid highlighter style name (e.g., :rouge-style: pastie).

Assuming all the required gems install properly, verify you can run the asciidoctor-epub3 script:

$ asciidoctor-epub3 -v

If you see the version of Asciidoctor EPUB3 printed, you’re ready to use Asciidoctor EPUB3. Let’s get an AsciiDoc document ready to convert to EPUB3.

The metadata in the generated EPUB3 file is populated from attributes in the AsciiDoc document. The names of the attributes and the metadata elements to which they map are documented in this section.

The term package metadata in Table 1 is in reference to the <metadata> element in the EPUB3 package document (e.g., package.opf). The dc namespace prefix is in reference to the Dublin Core Metadata Element Set.

AsciiDoc attributes that control the EPUB3 metadata (i.e., package.opf)
Name Description


Populates the required unique identifier (<dc:identifier>) in the package metadata. An id will be generated automatically from the doctitle if not specified. The recommended practice is to identify the document by means of a string or number conforming to a formal identification system.


Populates the content language / locale (<dc:language>) in the package metadata.


Controls the font subsets that are selected based on the specified scripts (e.g., alphabets). (values: latin, latin-ext, latin-cyrillic or multilingual)


Populates the publication date (<dc:date>) in the package metadata. The date should be specified in a parsable format, such as 2014-01-01.


Populates the title (<dc:title>) in the package metadata. The title is added to the metadata in plain text format.


Populates the contributors (<dc:contributor>) in the package metadata. The authors in each chapter document are aggregated together with the authors in the master file.


Used to resolve an avatar for the author that is displayed in the header of a chapter when the doctype is set to a value other than book. The avatar image should be located at the path {imagesdir}/avatars/{username}.jpg, where {username} is the value of this attribute.


Populates the publisher (<dc:publisher>) in the package metadata.


Populates the creator (<dc:creator>) in the package metadata. If the creator is not specified, Asciidoctor is set as the creator with the role "mfr" (an abbreviation for manufacturer).


Populates the description (<dc:description>) in the package metadata.


Populates the subjects (i.e., <dc:subject>) in the package metadata. The keywords should be represented as comma-separated values (CSV).


Populates the front cover image and the image on the cover page (EPUB3 only) in the package metadata. The image is also added to the e-book archive. The value may be specified as a path or inline image macro. Using the inline image macro is preferred as it allows the height and width to be specified.


Populates the rights statement (<dc:rights>) in the package metadata.


Populates the source reference (<dc:source>) in the package metadata. The recommended practice is to identify the referenced resource by means of a string or number conforming to a formal identification system.


An optional override of the properties attribute for this document’s item in the manifest. Only applies to a chapter document.


Specify the section level at which to split the EPUB into separate "chapter" files. This attribute only affects documents with :doctype: book. The default is to split into chapters at level-1 sections. This attribute only affects the internal composition of the EPUB, not the way chapters and sections are displayed to users. Some readers may be slow if the chapter files are too large, so for large documents with few level-1 headings, one might want to use a chapter level of 2 or 3.

series-name, series-volume, series-id

Populates the series statements (belongs-to-collection) in the package metadata. Volume is a number, ID probably a UUID that is constant for all volumes in the series.


The path to a directory that contains frontmatter files. The file names must match front-matter*.html and will be included in alphabetic order. The files are expected to be valid EPUB HTML files. If only one front matter page is required, the default 'front-matter.html' file can be used instead.


The path to a directory that contains alternate epub3.css and epub3-css3-only.css files to customize the look and feel.


Used to control the inclusion of special content in the generated HTML. If set to a value other than book, the byline information (author and avatar) is included below the chapter header and a typographic end mark is added at the end of the last paragraph. Suggested values include: book (default), article.


Adds table of contents at the beginning of the book. Depth is controlled by :toclevels: attribute.


Sets the depth of table of contents metadata. If not set, defaults to :toclevels:

When using the EPUB3 converter, the ebook-format attribute resolves to the name of the e-book format being generated (epub3 or kf8) and the corresponding attribute ebook-format-<name> is defined, where <name> is epub3 or kf8. You can use these attributes in a preprocessor directive if you only want to show certain content to readers using a particular device. For instance, if you want to display a message to readers on Kindle, you can use:

ifdef::ebook-format-kf8[Hello Kindle reader!]

With that out of the way, it’s time to convert the AsciiDoc document directly to EPUB3.

Performing the Conversion

You can convert AsciiDoc documents to EPUB3 and KF8/MOBI from the command line using the asciidoctor-epub3 script provided with the Asciidoctor EPUB3 project.

Convert AsciiDoc to EPUB3

Converting an AsciiDoc document to EPUB3 is as simple as passing your document to the asciidoctor-epub3 command. This command should be available on your PATH if you installed the asciidoctor-epub3 gem. Otherwise, you can find the command in the bin folder of the project. We also recommend specifying an output directory using the -D option flag.

$ asciidoctor-epub3 -D output data/samples/sample-book.adoc

When the script completes, you’ll see the file sample-book.epub appear in the output directory. Open that file with an EPUB reader (aka e-reader) to view the result.

Below are several screenshots of this sample book as it appears on an Android phone.

chapter title
Figure 2. An example of a chapter title and abstract shown side-by-side in Day and Night modes
section title paragraph
Figure 3. An example of a section title followed by paragraph text separated by a literal block
figure admonition
Figure 4. An example of a figure and an admonition
Figure 5. An example of a sidebar
Figure 6. An example of a table
The asciidoctor-epub3 command is a temporary solution for invoking the Asciidoctor EPUB3 converter. We plan to remove this script once we have completed proper integration with the asciidoctor command.
As another example, point asciidoctor-epub3 at the GitHub Guides that we’ve ported to AsciiDoc, then compare the output to the real GitHub Guides.

Validate the EPUB3 Archive

Next, let’s validate the EPUB3 archive to ensure it built correctly.

EPUB3 with validation
$ asciidoctor-epub3 -D output -a ebook-validate data/samples/sample-book.adoc
Validation success
Validating using EPUB version 3.0.1 rules.
No errors or warnings detected.
Messages: 0 fatal / 0 errors / 0 warnings / 0 info
EPUBCheck completed

If the EPUB3 archive contains any errors, they will be output in your terminal.

EPUB Standard & Validator

The electronic publication (EPUB) standard is developed by the International Digital Publishing Forum (IDPF). EPUB 3.1, released in January 2017, is the latest version of this standard.

An EPUB3 archive contains:

  • a package document (metadata, file manifest, spine)

  • a navigation document (table of contents)

  • one or more content documents

  • assets (images, fonts, stylesheets, etc.)

The IDPF also supports EPUBCheck. EPUBCheck parses and validates the file against the EPUB schema.

If you want to browse the contents of the EPUB3 file that is generated, or preview the XHTML files in a regular web browser, add the -a ebook-extract flag to the asciidoctor-epub3 command. The EPUB3 file will be extracted to a directory adjacent to the generated file, but without the file extension.

$ asciidoctor-epub3 -D output -a ebook-extract data/samples/sample-book.adoc

In this example, the contents of the EPUB3 will be extracted to the output/sample-book directory.

Convert AsciiDoc to KF8/MOBI

Creating a KF8/MOBI archive directly from an AsciiDoc document is done with the same generation script (asciidoctor-epub3). You just need to specify the format (-a ebook-format) as kf8.

$ asciidoctor-epub3 -D output -a ebook-format=kf8 data/samples/sample-book.adoc

When the script completes, you’ll see the file as well as sample-book-kf8.epub (the precursor) appear in the output directory.

KindleGen does mandatory validation so you don’t need to run the validate command after converting to KF8/MOBI.

What is KF8?

Kindle Format 8 (KF8) is Amazon’s next generation file format offering a wide range of new features and enhancements—​including HTML5 and CSS3 support—​that publishers can use to create a broad range of books. The format is close enough to EPUB3 that it’s safe to think of it simply as an EPUB3 implementation under most circumstances. You can read more about the format on the Kindle Format 8 page.

Amazon continues to use the .mobi file extension for KF8 archives, despite the fact that they’ve switched from the Mobipocket format to the EPUB3-like KF8 format. That’s why we refer to the format in this project as KF8/MOBI.

Tuning Listing Captions

Unlike the built-in converters, the EPUB3 converter is configured to add a signifier (e.g., Listing) at the start of the caption for all listing and source blocks that have a title. This behavior is triggered because the listing-caption attribute is set by default.

If you don’t want the signifier to be included at the beginning of the caption on listing and source blocks, simply unset the listing-caption when invoking Asciidoctor EPUB3.

$ asciidoctor-epub3 -a listing-caption! book.adoc

Now the behavior will match that of the built-in converters. For more information about this attribute and other related attributes, see internationalization and numbering.

Command Arguments

-h, --help

Show the usage message

-D, --destination-dir

Writes files to specified directory (defaults to the current directory)

-a ebook-epubcheck-path=<path>

Specifies path to EPUBCheck executable to use with -a ebook-validate. This attribute takes precedence over EPUBCHECK environment variable.

-a ebook-extract

Extracts the EPUB3 to a folder in the destination directory after the file is generated

-a ebook-format=<format>

Specifies the ebook format to generate (epub3 or kf8, default: epub3)

-a ebook-kindlegen-path=<path>

Specifies path to KindleGen executable to use when producing KF8/Mobi. This attribute takes precedence over KINDLEGEN environment variable.

-a ebook-validate

Runs EPUBCheck to validate output file against the EPUB3 specification

-a ebook-compress=<0|1|2|none|standard|huffdic>

Controls the compression type used by kindlegen (0=none [default if unset], 1=standard [default if empty], 2=huffdic)

-v, --version

Display the program version

Environment variables


Specifies path to EPUBCheck executable to use with -a ebook-validate. Effect of this variable can be overriden with -a ebook-epubcheck-path attribute.


Specifies path to KindleGen executable to use when producing KF8/Mobi. Effect of this variable can be overriden with -a ebook-kindlegen-path attribute.

EPUB3 Archive Structure

Here’s a sample manifest of files found in an EPUB3 document produced by Asciidoctor EPUB3.


Working with Images

Images referenced in your AsciiDoc document must be stored in the images catalog. The images catalog is defined by the imagesdir attribute. If set, the value of this attribute is resolved relative to the document and must be at or below (i.e., within) the directory of that document. (In other words, it cannot point to a location outside the document directory). If this attribute is not set, the images catalog defaults to the directory of the document. Asciidoctor EPUB3 will discover all local image references and insert those images into the EPUB3 archive at the same relative path.

Default Images

The sample book contains a placeholder image for the author avatar.

Adding the Cover Image

E-readers have different image resolution and file size limits regarding a book’s cover. Kindle covers tend to be 1050x1600 (16:9 aspect ratio). To ensure your cover displays correctly, you’ll want to review the documentation or publisher guidelines for the e-reading platform you’re targeting.

We’ve found that if the book cover is more than 1600px on any side, Aldiko will not render it and may even crash.

Feel free to use the SVG of the sample cover in the data/images folder as a template for creating your own cover. Once your image is ready, you can set the cover image by defining the front-cover-image attribute in the header of the master document.

:front-cover-image: image:cover.png[Front Cover,1050,1600]

The image is resolved relative to the directory specified in the imagesdir attribute, which defaults to the directory of the3 document. The image can be in any format, though we recommend using PNG, JPG, or SVG as they are the most portable formats.

You should always specify the dimensions of the cover image. This ensures the viewer will preserve the aspect ratio if it needs to be scaled to fit the screen. If you don’t specify a width and height, then the dimensions are assumed to be 1050x1600.

How to Organize Images by Chapter

You can set the imagesdir attribute by chapter (as long as the attribute is not overridden by the API). To do so, use an attribute entry to set the value of the imagesdir attribute on the line above the include directive for a chapter.

:imagesdir: chapter-one/images

:imagesdir: chapter-two/images

About the Theme

EPUB3 and KF8/MOBI files are styled using CSS3. However, each e-reading platform honors a reduced set of CSS3 styles, and the styles they allow and how they implement them are rarely documented. All we’ve got to say is thank goodness for CSS hacks, media queries and years of CSS experience!

The theme provided with Asciidoctor EPUB3 has been crafted to display EPUB3 and KF8/MOBI files as consistently as possible across the most common EPUB3 platforms and to degrade gracefully in select EPUB2 platforms. The theme maintains readability regardless of the reading mode (i.e., day, night or sepia) or the display device’s pixel density and screen resolution.

The theme’s CSS files are located in the data/style directory.

Asciidoctor EPUB3 only provides one theme, and, at this time, you can not replace it with a custom theme using the stylesheet attribute. However, you can use your own epub3.css and epub3-css3-only.css files by specifying the directory where they are located using the epub3-stylesdir attribute.

Device-specific Styles

For readers that support JavaScript, Asciidoctor EPUB3 adds a CSS class to the body element of each chapter that corresponds to the name of the reader as reported by the epubReadingSystem JavaScript object. This enhancement allows you to use styles targeted specifically at that reader.

Below you can find the readers that are known to support this feature and the CSS class name that gets added to the body element.

Reader HTML Element CSS Class Name




Namo PubTreeViewer









Adobe RMSDK >= 11



Google Books



Kobo does not support the epubReadingSystem JavaScript object, despite the fact that it does support JavaScript.

Pushing to Android

While it’s certainly possible to view the EPUB3 on your desktop/laptop, you’ll probably want to test it where it’s most likely going to be read—​on a reading device such as a smartphone or a tablet. Assuming you have an Android device available, transferring the EPUB3 to the device is easy once you get a bit of setup out of the way.

You transfer files from your computer to an Android phone over a USB connection using a command from the Android SDK Tools called adb. Follow these steps to get it set up:

  1. Download the Android SDK Tools zip from the table labeled SDK Tools Only on the Get the Android SDK page

  2. Extract the archive

  3. Locate the path to the adb command (Hint: Look in the platform-tools folder)

  4. Set the environment variable named ADB to the path of the adb command

    $ export ADB=~/apps/android-sdk/platform-tools/adb

Now you can use the adb-push-ebook script provided by Asciidoctor EPUB3 to push the EPUB3 and KF8/MOBI files to your Android device.

Publish both EPUB3 and KF8 files to Android device
$ adb-push-ebook output/sample-book
Don’t include the file extension since the script will check for both .epub and .mobi files.

The adb-push-ebook script copies the files to the following locations on the device:

File type Destination on Android device





Amazon Kindle should immediately detect the new file and display it in your “On Device” library. You’ll have to manually import the EPUB3 into your e-reader of choice.

E-book Reader Recommendations and Quirks

EPUB3 e-readers will provide the best reading experience when viewing a book generated by Asciidoctor EPUB3. Here’s a list of some of the e-readers we know to have good EPUB3 support and the systems on which they run:

To get the full experience, ensure that the e-reader is configured to use the publisher’s styles. Different e-readers word this setting in different ways. Look for the option screen that allows you to set the fonts and font colors and disable it. With publisher’s styles active, you’ll still be able to adjust the relative size of the fonts and margins and toggle between day, night and sepia mode.

When the book is viewed in EPUB2 e-readers and Kindle apps/devices which have reached their end-of-life (EOL), the e-book relies on the strong semantics of HTML and some fallback styles to render properly. EPUB2 e-readers, such as Aldiko, don’t understand CSS3 styles and therefore miss out on some of the subtleties in the formatting.

As mentioned in the theme section, the stylesheet attempts to provide as consistent a reading experience as possible in the common EPUB3 e-readers, despite the different CSS implementation rules and limitations unique to each e-book application. Most of these obstacles were addressed using media queries or explicit classes. Some we haven’t conquered. Yet.

The Kindle quirks list shows you just a few of the constraints we encountered. To see all of the workarounds and why we chose certain style options, check out the code and comments in the epub3.css and epub3-css-only.css files.

Kindle Quirks
  • overrules margins and line heights like a medieval tyrant

  • font-family can’t be set on <body>

  • requires !important on text-decoration

  • position: relative isn’t permitted

  • strips (or unwraps) <header> tags

  • @page isn’t supported

  • page-break: avoid isn’t supported

  • page-break-* cannot be applied using a compound or nested CSS selector; must be a simple ID or class

  • max-width isn’t supported

  • widows are left in the cold

  • won’t style footers without an explicit class

  • -webkit-hyphens: auto causes Kindle for Mac (and perhaps others) to crash

  • text-rendering: optimizeLegibility causes file to be rejected by KFP (and causes the text to disappear in some previewers)

  • Kindle Direct Publishing (KDP) strips out select font-related CSS rules (e.g., font-family) under certain conditions (for reasons that have proved nearly impossible to reverse engineer); the known workaround is to add a layer of indirection by using @import to hide the CSS files from the script

Kindle Direct Publishing

If you want to publish your book to the Amazon Kindle store, and have your styles and fonts preserved, you must use Kindle Direct Publishing. No other method of publishing to the Amazon Kindle store will leave your book intact.

This workflow also allows you to preview the book the way your readers will see it. So it’s a good way to acceptance test your custom styles to discover which ones are honored and which ones are ignored.

Look Inside

The Amazon Kindle store offers a “Look Inside” feature for reader. This so-called feature aims to gives readers a glimpse at the contents of the book. Sadly, it does no such thing. Instead, it shows pages from the legacy MOBI document that kindlegen adds to your e-book file, not the higher fidelity KF8 document. This means the preview won’t look at all like what the Kindle reader or application will actually show. Custom fonts, font-based icons, and most styles will be missing.

This situation is unfortunate for authors. What we recommend is to complain loudly to Amazon that they are hurting your sales by displaying a crippled version of your product. Tell them that under no circumstances should they show an altered version of your product. Another option is to contact Amazon to opt-out of this program.

Send to Kindle

Don’t use it!

Send to Kindle is a tempting choice for transferring MOBI files to a Kindle device. However, it’s utterly broken. If you use this tool, don’t be surprised if you see missing font-based icons, default fonts, and other font and style errors in your manuscript. It’s known to strip out all the font files and break the encoding of the document.

We strongly recommend you transfer the file to your device using either a USB cable or a sync service such as Dropbox. Once transferred, Whispersync will detect the new file (usually looking in the Books folder) and add it to the “On Device” library.

It’s important to note that “Send to Kindle” is not reflective of the experience readers will have when shopping in the Kindle store. If you use Kindle Direct Publishing (KDP) to publish your book, the integrity of your book will be preserved (to the degree that Amazon allows).