bootstrap-sass, Confusion that define white-space twice time

Question

code {
   padding: 0;
   color: inherit;
   white-space: pre;              // first time
   white-space: pre-wrap;        // why do this twice?
   background-color: transparent;
   border: 0;
}

it's in /vendor/assets/stylesheets/bootstrap/_code.scss

why not just define white-space: pre-wrap, from my point of view, it works well.
Does it work for some special purpose?

Answer 1

pre-wrap is not availabe in all browsers and pre is its backup.