You can see which features are supported by WebView with this code:
B4X:
Sub AppStart (Form1 As Form, Args() As String)
MainForm = Form1
MainForm.SetFormStyle("UNIFIED")
MainForm.Show
Dim wv As WebView
wv.Initialize("")
MainForm.RootPane.AddNode(wv, 0, 0, 500, 500)
wv.LoadUrl("https://html5test.com/")
End Sub
It doesn't support the File API which is apparently required by your scripts.