My frustrating experience was in FMP8.
Most important, when editing or creating a script, there is a checkbox "indicate web compatibility". Will dim scriptsteps that are not compatible.
Second, fields sometimes need to be slightly bigger on the WebView Layout to display correctly (especially checkboxes and radio buttons).
Different browsers on different OS's will display differently.
Most of the info on formatting an IWP Layout, gave generalities and "try different things" advice.
I would look at Links 4, 5, 6, 8, and 9 on the first page of this Google. Especially, the posts on the IWP Subforum.
Important Images can either be stored inside the database itself or, for best performance, stored as file reference using a relative path. If you’re hosting the database with FileMaker Pro, referenced image and movie files must be stored in the Web folder, in the FileMaker Pro folder.