Looking at your profile you already are familiar with Xpages.
If the app is not to complex i would go for Xpages. Since styling is easier then in a trad. notes app. For me that is.
Here is a example of lotusscript which makes section in a memo since i am not sure what kind of app you are making and for what platform, notes, web or both.
On breaking par is a example wich uses mailfile an memo form and places section and text in mail body
Here is the code
Sub Initialize
Dim session As New NotesSession
Dim mailDb As New NotesDatabase("", "")
Dim ws As New NotesUIWorkspace
Dim doc As NotesDocument
Dim body As NotesRichTextItem
Dim style As NotesRichTextStyle
Dim color As NotesColorObject
Call mailDb.OpenMail
Set doc = mailDb.CreateDocument
Call doc.ReplaceItemValue("Form", "Memo")
Set body = doc.CreateRichTextItem("Body")
Set style = session.CreateRichTextStyle
Set color = session.CreateColorObject
Call body.AppendText("This is some text before the section")
Call body.AddNewline(2)
Call body.BeginSection("Expanded Section", style, color, True)
Call body.AppendText("Here is some text within the section")
Call body.AddNewline(2)
Call body.AppendText("Here is some more text within the section")
Call body.EndSection
Call body.AddNewline(2)
Call body.AppendText("This is some text between the two sections")
Call body.AddNewline(2)
Call body.BeginSection("Collapsed Section")
Call body.AppendText("Here is some text within the section")
Call body.AddNewline(2)
Call body.AppendText("Here is some more text within the section")
Call body.EndSection
Call body.AddNewline(2)
Call body.AppendText("This is some text after the section")
Call doc.Save(True, False, False)
Call ws.EditDocument(True, doc)
Call doc.Remove(True)
End Sub
For completness here is a tutorial how to call a lotusscript agent from a xpage
Tutorial-Introduction-to-XPages-Exercise-20
Hope it helps.
Edit:
Just remembered they held a survey about css and html in the notes client.