But which mechanism would do the substitution of those parameters in the HTML of the child frame? I still fail to see how it would be done without either Javasc

But which mechanism would do the substitution of those parameters in the HTML of the child frame? I still fail to see how it would be done without either Javascript or server-side code. (And I did web dev in the 90s, with frames, but apparently was not evil enough to think of such stuff.)