1. Built-in CSS rules

XMLmind XML Editor has built-in CSS rules mainly used to style comments and processing instructions. These built-in rules are always implicitly loaded before the rules found in a CSS file. However, nothing prevents you from overriding any of the following built-in rules.

/*
 * Copyright (c) 2017 XMLmind Software. All rights reserved.
 *
 * This file is part of the XMLmind XML Editor project.
 * For conditions of distribution and use, see the accompanying legal.txt file.
 *
 * Built-in CSS rules.
 */

/* ------------------------------------------------------------------------ 
   Comments and processing-instructions
   ------------------------------------------------------------------------ */

*::comment,
*::processing-instruction {
    display: inherit;
    white-space: pre;
    text-align: left;
    font-family: monospace;
    font-style: normal;
    font-weight: normal;
    font-size: small;
    margin: 0.25ex;
}

*::comment {
    background-color: #FFFFDD;
    color: #606000;
}

*::processing-instruction {
    background-color: #DDFFDD;
    color: #006000;
}

/* Do not report an error if the "Spreadsheet" add-on has not been installed. 
   (This should be the only place in the core code where XXE knows 
    about the Spreadsheet add-on.) */

@import url(---spreadsheet:spreadsheet.imp);

/* ------------------------------------------------------------------------ 
   Remark
   ------------------------------------------------------------------------ */

*::processing-instruction(xxe-remark) {
    display: inline;
    content: gadget("com.xmlmind.xmleditapp.cmd.diff.RemarkIndicator");
    font-size: inherit;
    background-color: transparent;
}

/* ------------------------------------------------------------------------ 
   Nodes marked read-only
   ------------------------------------------------------------------------ */

@namespace prop "http://www.xmlmind.com/xmleditor/namespace/property";

*:property(prop|readOnly, "true") {
    background-color: #E0F0F0;
}

*:property(prop|readOnly, "false") {
    background-color: normal;
}

/* ------------------------------------------------------------------------ 
   XInclude directive
   ------------------------------------------------------------------------ */

@namespace xi url(http://www.w3.org/2001/XInclude);

@property-value xi-include-content() 
    paragraph(content(icon(pop-se, 0), " ", element-name(), 
                     " href=", attr(href),
                     " xpointer=", xpath("if(@xpointer, @xpointer, @fragid)")));

/* The xpointer attribute must not be present when parse="text" */

@property-value xi-text-include-content() 
    paragraph(content(icon(pop-se, 0), " ", element-name(), 
                      " href=", attr(href),
                      " fragid=", attr(fragid),
                      " parse=", attr(parse),
                      " encoding=", attr(encoding)));

xi|include {
    display: block;
    margin-bottom: 1.33ex;
}

xi|include:before {
    display: block;
    content: xi-include-content();
    font-style: italic;
    color: gray;
    margin-top: 1.33ex;
}

xi|include[parse*="text"]:before {
    content: xi-text-include-content();
}

xi|include:empty {
    display: inherit;
    content: xi-include-content();
    font-style: italic;
    color: gray;
}

xi|include:empty:before {
    content: "";
}

xi|include[parse=text]:empty {
    content: xi-text-include-content();
}

xi|include[parse=text]:empty:before {
    content: "";
}

xi|fallback {
    display: block;
    font-family: normal;
    font-size: medium;
    font-weight: normal;
    font-style: normal;
    background-color: normal;
    color: normal;
    padding: 0.5ex;
    border: 1px dotted gray;
    margin: 0.5ex;
}

/* ------------------------------------------------------------------------ 
   When printing
   ------------------------------------------------------------------------ */

@media print {
    * {
        line-height: 1.2; 
    }

    *::comment,
    *::processing-instruction {
        display: none;
    }

    *:property(prop|readOnly) {
        background-color: transparent;
    }
}

In practice, this just means that you have nothing special to do to style nodes marked as being read-only, XIncludes, comments, processing instructions and spreadsheet formulas (processing instruction <?xxe-formula>).