Is there a way to create a custom edit item page for SharePoint list items using HTML/CSS/JS? I want to be able to modify EditItem.aspx so that I could add divide the fields into different sections and insert buttons on the page. I've tried to using Mark Rackley's solution for this task but I am unable to pre-populate the fields in my HTML page with existing data.
Option 1: Does it have to be entirely in SharePoint? Why not build the page out using exactly the things that you know, HTML/CSS/JS and make all of your connections to a SharePoint list?
Option 2: Edit the form in SharePoint designer.