Right-click a html file and print it?
This is pretty easy to do through automator and the "Watch me do" actions. Here's what I got to work however you'd likely replace your input with however you're getting the file in your setup



Display Webpages opens the HTML file in the default browser (in this case chrome) and then we enter Command-P and then Enter which brings up the print dialog and then prints with the default settings because the print button is the initial highlighted button. This will work with whatever fancy javascript you've got and you can optionally add a wait period between launching the file and sending the print keys to allow it to fully render/load.