Chummer uses XSLT for all character printouts with the idea that users can create their own character sheets to suit specific needs if desired. When the printout window opens, it lists all of the XSL files located in the sheets directory in a dropdown list. The list only includes files that end in ".xsl"; it ignores files ending in ".xslt" which can be used as partial template files, thus hiding files that should not be used on their own. All custom sheets should be placed in Chummer's \sheets directory.
In order to create a custom character sheet you must be familiar with XSL Transformation markup. Of course this isn't helpful unless you have an XML file to examine to understand the structure. You can view a character's output XML by opening the Character Viewer window and clicking the Save XML button.
Because there are a number of special characters throughout the data files and content can be in other languages besides English, it is strongly recommended that you save your XSL file with UTF-8 encoding. You should also take the following steps to ensure your output is encoded correctly on other computers:
The beginning XML tag in your XSL file should specify UTF-8 encoding.
<?xml version="1.0" encoding="UTF-8" ?>
You should also include a meta tag in your output that specifies the Content-Type as UTF-8.
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
Once you have created your sample XML file using the Save XML button, you can experiment with your character sheet file outside of Chummer with my XSLTransform Tool