Multiple Selections in List Boxes on Web Forms

You must use the Ctrl key or Shift key to select multiple items in a list box on a Web form, as HTML does not support the MultiSelect_Simple functionality. Multiple selections are therefore regarded as extended multiple selections (that is, MultiSelect_Extended, where the Shift+click or Shift+arrow key extends the selection from the previously selected item to the current item or Ctrl+click selects or deselects an item in the list).