The hl_tcl package is a syntax highlighter for Tcl/Tk code.
It can be applied to a Tk text widget or to a static html page.
The Tk text widget may be made read-only or editable. Also, the hl_tcl may take an argument, sort of command to watch the viewing / editing.
When applied to html pages, the hl_tcl highlights Tcl/Tk code snippets embedded between <code> </code> tags.
The hl_tcl has highlighted its own code in Reference.