TWebDBGrid

It should inherrit everything from TWebStringgrid but
result := WebDBGrid.Cells[col,row] do not work so how to do it

It should work. What exact problem are you experiencing?

When I retest this in demo \Demo\Basics\DBGrid with code added

procedure TForm1.WebButton1Click(Sender: TObject);
begin
  showmessage(webdbgrid1.cells[1,webdbgrid1.Row]);
end;

this shows the expected value.

It was an oversight for LSP but this code will compile & work.
We'll look to overcome this LSP limitation.