Determines the tab order of the weblet on the form. The tab_index property value determines the tab order as follows:
1. Objects with a positive tab_index are selected in increasing tab_index order (and in source order to resolve duplicates).
2. Objects with a tab_index of zero or blank (the default) are selected in source order.
3. Objects with a negative tab_index are omitted from the tabbing order. Note that this behavior is not defined in the HTML specifications and is only supported by Internet Explorer and Firefox.
Blank. The weblet is selected in source order.
Blank or a valid numeric value.