Five Interesting Things

Norman Tovey-Walsh

This book highlights five interesting things about the DocBook xslTNG Stylesheets.

Revision History
1.1.018 Jul 2020ndw

Renamed chapter 1 and chapter 5 on the advice of a reviewer.

1.0.129 Jun 2020ndw

Small changes motivated by improvements in the CSS for paged media.

1.0.028 Jun 2020ndw

First publication.

0.0.120 Jun 2020ndw

Initial draft.

Chapter 1. Clean Look and Feel

The output from the DocBook xslTNG Stylesheets is designed to be clean, semantic HTML with accessibility features wherever possible. The online presentation is driven by modern, widely-available CSS and JavaScript APIs.

Screenshot of the new look and feel as rendered in Firefox. There’s a header across the top with Home, Up, Next, and Previous links. There’s a footer across the bottom with similar navigation features divided into a three column table: Previous on the left, Up and Home in the center, and Next on the right. The titles of preceding and next pages are shown in the footer.

Screenshot of this chapter as rendered in Firefox on the Mac
Figure 1.1 Look and Feel in Firefox

The header is fixed to the top of the page so navigation links are always available.

Accessibility features include long descriptions of images and tables, alt-text for images, and headers for table rows and columns. Where JavaScript is used, reasonable non-JavaScript fallbacks are provided.

On short pages, the footer always floats to the bottom of the page.

The “persistent ToC” feature puts the whole document’s Table of Contents within easy reach on every page.

The design is generally cleaner. There are different and slightly larger fonts, the column width never exceeds a practical reading width, and media queries make the design responsive to the narrower displays on phones and tablets.

The focus on clean, semantic markup means that you can create new (and better!) styles with CSS. You can also write stylesheet extensions to customize the markup, to add additional details to the header and footer, for example.

Chapter 2. Persistent Table of Contents

The Persistent Table of Contents brings the document’s Table of Contents navigation hierarchy to every page. Clicking on the “[toc]”1 link in the upper right corner slides out the Table of Contents window from the right hand edge of the screen.

Screenshot showing the persistent table of contents rendered as a popup on the right hand side of the page.

Screenshot of this chapter with the persistent ToC as rendered in Firefox on the Mac
Figure 2.1 Persistent Table of Contents in Firefox

Links in the persistent ToC navigate to the pages they identify. For large documents with long ToCs, the window scrolls and attempts to place the current page at the top of the ToC view.

Chapter 3. Callouts

Callouts are a mechanism for decorating the verbatim content of programlisting elements with marks that can be cross-referenced in documentation. They have the real advantage that they can be used to decorate the actual code or other listing. Importing the actual listing assures that the documented listing and the actual listing will be the same.

A system where you have to import the code or add the markup inline introduces the risk that the listing being documented will not be updated when the underlying resource is updated.

The program below computes the “nth” Fibonacci number.

1#!/bin/env python
"""Computes the n'th Fibonacci number."""
5import sys
import math
import click  
SQRT5 = math.sqrt(5)  

Click is my current favorite library for parsing command line arguments in Python.

This constant1 is the 5.

I’m also taking advantage of the ability to use XInclude (and RFC 5147) to break the listing into manageable fragments for documentation. The listing above, and the one below, are lines from the same file.

@click.argument("number", nargs=1,
                type=click.IntRange(1, None))
def fibonacci(number):
15    """Calculate the 'number'th Fibonacci number."""
    prefix = f"The {make_ordinal(number)} Fibonacci number is "
        value = (((1 + SQRT5) / 2) ** number -
                 ((1 - SQRT5) / 2) ** number) / SQRT5      
20        print(f"{prefix}{math.floor(value):,}.")
    except OverflowError:
        print(f"{prefix}too large to calculate.")
def make_ordinal(n):
    """Create an ordinal from a number."""
    suffix = ["th", "st", "nd", "rd", "th"][min(n % 10, 4)]
30    if 11 <= (n % 100) <= 13:
        suffix = "th"
    return str(n) + suffix
35if __name__ == "__main__":

One of the real strengths of using a tool like Click to wrangle arguments is that you get type and range checking pretty much for free.

This is the closed-form solution to finding the “nth” Fibonacci number:

1 5 1 + 5 2 n -1 -5 2 n

The closed-form solution amuses me because I used calculating Fibonacci numbers (recursively) as an argument for greater expressive power in XSLT 1.0. “What if I wanted to number a list with the Fibonacci numbers?”

Stackoverflow showed me how to print the number as an ordinal.

Finally, if this is a main program, compute the answer.

If you hover over the callout numbers in the listing, alt-text will be displayed.

Chapter 4. Annotations

Annotations, as the name implies3, allow you to associate annotations with content in your document. Annotations are non-local (they’re associated by ID/IDREF not containment) and the linking markup is bi-directional4. The same annotation can2 be used more than once2.

Annotations can be rendered with or without JavaScript support. With JavaScript, the annotations are popups. Without JavaScript, they’re presented at the bottom of the page, similar to footnotes, but styled somewhat differently. In paged media, annotations are rendered as footnotes and their titles are ignored.

Appendix A. Paged media support

The DocBook xslTNG Stylesheets anticipate that “print” output, that is, paged media, will be provided with a CSS formatter such as AntennaHouse or Prince.

The stylesheet intended for paged media output produces slightly different HTML than the online stylesheet. This print customization assures that JavaScript features are disabled and changes the markup used for some elements.

The resulting “.pdf.html” file is then processed by the formatter.

In as much as the different CSS formatters behave differently and have different extensions for features that are needed in print but not yet standardized, additional customization may be necessary.


[AntennaHouse] AH Formatter. Version 7.0.3.

[CanIUse] Can I use… Support tables for HTML5, CSS3, etc. 20 June 2020.

[Click] Click. Version 7.2.

[CSS] CSS Specifications. World Wide Web Consortium. 2020.

[HTML] HTML: Living Standard. 18 June 2020.

[MathJax] MathJax.

[MathML] Mathematical Markup Language (MathML) Version 3.0. World Wide Web Consortium. 2014.

[metadata-extractor] metadata-extractor. Version 2.14.0.

[Prince] Prince. Version 13.5.

[RFC 5147] URI Fragment Identifiers for the text/plain Media Type. E. Wilde and M. Duerst, editors. Internet Engineering Task Force. 2008.

[XInclude] XML Inclusions (XInclude) Version 1.0 (Second Edition). World Wide Web Consortium. 2006.


This book was authored in DocBook 5.1 with Emacs. It was formatted for presentation with the DocBook xslTNG Stylesheets.