CSS for Shallow Numbering
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There is a file located in [PLUGIN_DIR]/css/print/p-numbering-shallow. The following examples are from this file.

Note: This is listed solely for illustration purposes, as the plugin might use something different.

The counters should be reset on the root element. This is the element that has the class map/map. Note that bookmaps derive from maps and the following selector will also match them:

*[class ~= "map/map"] {
    counter-reset: page 1
                     toc-chapter 0
                     chapter 0
                     figcount 0
                     tablecount 0;
}

Table of Contents

There is only one counter for the chapters, named toc-chapter. This is reset on the book parts (for bookmaps) and is incremented on each chapter that is not a part.
Note: The parts are also marked as chapters.
*[class ~= "map/topicref"][is-chapter]:not([is-part]) {
    counter-increment:toc-chapter;
}
*[class ~= "map/topicref"][is-part] {
   counter-reset:toc-chapter !important;
   counter-increment:toc-part;
} 
 
/* All other topicrefs that follow a part should not have a number (i.e appendices)  */
*[class ~= "map/topicref"][is-part] ~ *[class ~= "map/topicref"]:not([is-part]) > *[class ~= "map/topicmeta"] > *[class ~= "topic/navtitle"]:before{
   content: none !important;
   counter-increment:none;
}
To display the numbers in the TOC, a :before pseudo element is used on the navigation titles from the <topicref> elements:
*[class ~= "map/topicref"][is-part] > *[class ~= "map/topicmeta"] > *[class ~= "topic/navtitle"]:before{
   content:"Part " counter(toc-part, upper-roman) ". " !important;
   color:inherit;
}    
*[class ~= "map/topicref"][is-chapter]:not([is-part]) > *[class ~= "map/topicmeta"] > *[class ~= "topic/navtitle"]:before{
    content:"Chapter " counter(toc-chapter) ". " !important;
    color:inherit;
}

The Titles of Chapters in the Main Content

The part and chapter counters are incremented on the elements that have the class topic/topic:
*[class ~= "topic/topic"][is-part]{
    counter-increment:part;
    counter-reset:chapter;
}
*[class ~= "topic/topic"][is-chapter]:not([is-part]){
    counter-increment:chapter;
}
Except for <frontmatter>, <backmatter>, and other topics following the last book part (for bookmaps).
*[class ~= "bookmap/frontmatter"],
*[class ~= "bookmap/frontmatter"] *[class ~= "topic/topic"],
*[class ~= "bookmap/backmatter"],
*[class ~= "bookmap/backmatter"] *[class ~= "topic/topic"],
*[class ~= "topic/topic"][is-part] ~ *[class ~= "topic/topic"]:not([is-part]) {
   counter-increment:none !important;
   counter-reset: chapter part;
}
These counters are used in the content, before the titles: 
*[class ~= "topic/topic"][is-part] > *[class ~= "topic/title"]:before {
    content: "Part " counter(part, upper-roman) ". ";
}
*[class ~= "topic/topic"][is-chapter]:not([is-part]) > *[class ~= "topic/title"]:before {
    content: "Chapter " counter(chapter) ". ";
}
Except for <frontmatter>, <backmatter>, and other topics following the last book part (for bookmaps).
*[class ~= "bookmap/frontmatter"] > *[class ~= "topic/title"]:before,
*[class ~= "bookmap/frontmatter"] *[class ~= "topic/topic"] > *[class ~= "topic/title"]:before,
*[class ~= "bookmap/backmatter"] > *[class ~= "topic/title"]:before,
*[class ~= "bookmap/backmatter"] *[class ~= "topic/topic"] > *[class ~= "topic/title"]:before,
*[class ~= "topic/topic"][is-part] ~ *[class ~= "topic/topic"]:not([is-part]) > *[class ~= "topic/title"]:before,
*[class ~= "topic/topic"][is-part] ~ *[class ~= "topic/topic"]:not([is-part]) *[class ~= "topic/topic"] > *[class ~= "topic/title"]:before{
    content:none !important;
}

Headers

Define the string-sets used in the header, using the part and chapter counters:
*[class ~= "topic/topic"][is-part] > *[class ~= "topic/title"] {
    string-set: parttitle " | " counter(part, upper-roman) " - " content(), chaptertitle  ""; /* Avoid propagating a past chapter title on a new part */
}
*[class ~= "topic/topic"][is-chapter]:not([is-part]) > *[class ~= "topic/title"] {
    string-set: chaptertitle " | " counter(chapter) " - " content();
}
Except for front matter, backmatter and other topics following the last book part (for bookmaps).
*[class ~= "bookmap/frontmatter"] > *[class ~= "topic/title"],
*[class ~= "bookmap/frontmatter"] *[class ~= "topic/topic"] > *[class ~= "topic/title"],
*[class ~= "bookmap/backmatter"] > *[class ~= "topic/title"],
*[class ~= "bookmap/backmatter"] *[class ~= "topic/topic"] > *[class ~= "topic/title"],
*[class ~= "topic/topic"][is-part] ~ *[class ~= "topic/topic"]:not([is-part]) > *[class ~= "topic/title"],
*[class ~= "topic/topic"][is-part] ~ *[class ~= "topic/topic"]:not([is-part]) *[class ~= "topic/topic"] > *[class ~= "topic/title"]{
   string-set: parttitle "", chaptertitle "";
}