Thank you for your post.
At this time, FileMaker Pro will not trigger a OnObjectModify script based on the contents of a Web Viewer, since you are not changing the object itself, but the contents of the web pages being rendered.
I have forwarded this to our Development and Software Quality Assurance (Testing) departments for review, but I would also encourage you to enter this into our Feature Requests web form at:
These web entry suggestions are read by our Development and Product Management departments where the ideas are then discussed and considered for future releases. Although I could copy your post and paste it into the web form, there are some additional questions asked that only you can answer.