Thinking about it there are so many angles from which one could approach it.

It could not be required to run the script. For example require the visitor has his own website and have the script interact with specified tools installed there. Or a very limited language that disables js for security.

It could be a highly readable DSL, for example something to write interpreters in.

Or something narrow like interact with a rich text editor or data visualizations.

Perhaps a basic game engine.