Web App options should be added to Connect, creating a single development app that accommodates both the Field App and Web App.
Presently, if you develop a survey in Connect that is either wholly or partly intended to be used in the Web App, you need to publish the survey, go to the S123 Website's Design tab, and then make additional changes to your survey (E.g., "Action after submission") and to preview the survey.
This new design mode should be able to swap the "Field App appearance" for the "Web App appearance" so you can preview the app offline, and the "Analyze survey" tool should give Web App hints (as there isn't complete parity between the two styles).