What is the proper value of the multiple attribute for select tag?

http://www.tizag.com/htmlT/htmlselect.php says "yes"

while

http://www.w3schools.com/tags/att_select_multiple.asp says "multiple"

I did, however, tried both and worked just fine but I want to be using what's acceptable by w3c to ensure full compatibility in most major browsers.

Why not read the official documentation instead of those websites. It states multiple has to be set, it is a boolean attribute, so it does not need a value.