Usually thermal print receipts wont support HTML, they support ESC/POS Commands
Do you can check that into the manual, of one of thermal printers of this vendor you need to send ESC/POS commands to the printer.
You may can use GitHub - techmaxsolucoes/py-xml-escpos: Print XML formated receipts on ESC/POS printers, in Python. to program your own print dumps to the printer, or make use of Print Node Integration for Frappe Framework
That already supports the generation of ESC/POS commands to thermal printer from an jinja template using XML ESC/POS syntaxis