Scripts do execute in IWP though some steps are not compatible. Script triggers do not respond to user actions like they do with Regular FileMaker. You can open your database file on your computer, turn on web publishing and then use a browser on the same computer to look at it. This is the quickest way to see what the layout looks like and how it will work in IWP.
In the script editor there's a compatibility drop down you can use to tell if the steps in your script are web compatible. There's a PDF that you can read on web publishing located in the Product Documentation section of your help file. That's where I figured out how to test a file like I mentioned in the first paragraph.