With Isadora 4 you could use Python for this.
With Isadora 3 the only way I can think to do it the way you want to would be some kind of clunky setup with an Automator script to open the webpage, Watch Folders, and using Data Array to write things into a text file in the folder that Automator is watching to trigger different scripts. The tricky part is getting it working on a Touch TV to allow the viewer to scroll the webpage without being able to mess with Isadora.
If at all possible it'd be much easier to limit the viewer's ability to mess with Isadora by putting Isadora on a non-touchscreen display and the webpage on its own touchscreen display. That way you could use the touchscreen display as a Stage for Isadora and just show a blank black stage (or whatever else you wanted) before using the Show-Hide Stages actor to programmatically hide the Stage on the touchscreen display, revealing the webpage (which you'd have already have had open on that display ahead of time).
The other option I can think of is to pre-record the webpage with the process journal using OBS or Isadora's Screen Capture actor + Capture Stage to Disk actor so that you have a video file of the webpage scrolling at a rate you're happy with. It would no longer be interactive, but it would be easier and self-contained this way.