Incorporating a select field into a web page is done using the <select> tag. List values are then added to the field using the <option> tag, similar to how list items <li> are added to ordered list elements (<ol>).
By default, select fields, popularly called drop down lists, only allow the user to choose a single value. This behavior and appearance may be changed by adjusting the multiple and size attributes as demonstrated below.
With the above settings, the user is now able to select multiple values by pressing and holding the Control (ctrl) key and clicking each value.
HTML - Disabling Selection Fields
Disabling a selection field is achieved by setting the disabled attribute to "yes". But before doing that, you should set at least one of the values to be selected. Doing so renders a read-only selection field on the page that can inform your users of their selections without allowing them to alter the selection.