Syntax highlighting

The XSLT 1.0 stylesheets use XSLTHL [XSLTHL], which is an extension function implemented for multiple XSLT processors and which uses XML data files. However, as noted in [GRAHAM2021], XSLTHL development has stalled and new languages are rarely added. The xslTNG stylesheets use Pygments, which is an external Python program but which covers a large number of languages and which is still actively developed.