The -X (--XSL) switch is used to specify an “extra” XSL stylesheet that will be applied to your source. Note that it is now possible to do anything you want to your source and so can create run-time failures at any point in the process. Here are the simple mistakes to avoid.
• If you are expecting HTML output because you used -f html, then your stylesheet supplied in the -X argument will certainly import the base xsl/pretext-html.xsl stylesheet. When you understand this, then you will understand that perhaps we should call this an “alternate” stylesheet. This advice applies equally well to extending the base xsl/pretext-latex.xsl stylesheet.
• The import just described will behave better for others (meaning co-authors, or authors who fork your project) if the import uses a relative path, meaning relative to location of the extra stylesheet. We therefore suggest using a user directory, placed as a peer of the xsl directory, as described in Section 28.2.
• So a common mistake is to use -X to point to an extra stylesheet someplace close-by your project's source files, when you have sensibly setup a relative import of the base stylesheet, and instead should point to the copy you have placed in user so that the import is effective on everybody's system.