A drop-down list that allows multiple selections:
The multiple attribute is a boolean attribute.
When present, it specifies that multiple options can be selected at once.
Selecting multiple options vary in different operating systems and browsers:
Because of the different ways of doing this, and because you have to inform the user that multiple selection is available, it is more user-friendly to use checkboxes instead.
In XHTML, attribute minimization is forbidden, and the multiple attribute must be defined as <select multiple="multiple">.