Don't recall this silent printing method in the Rebol documentation.
First, one needs JS Print Setup extension for Firefox.
Then, add this to the tag: onload=jsPrintSetup.print()
If it does not print when the page is loaded, Firefox silent printing must be enabled.
http://www.blueboxonline.com/?showkm&global[uid]=392-%28%29-Enabling-silent-printing-in-Firefox has the instructions, thanks!
When set, do browse %print.html in Rebol. The page will print silently. No buttons to push, no dialogues to negotiate. And the parameters of the Java script code can be modified with Rebol too.