What does !default in a css property value mean?

Question

The twitter bootstrap code has a lot of CSS properties with a !default at the end.

E.g.

p {
  color: white !default;
}

What does !default do?

UPDATE

My bad for not being clear. I am using the SASS part of Bootstrap.

Answer 1

!default is used often in Bootstrap Sass. It is similar to a reverse !important. All of Bootstraps Variables are set using !default to allow the developer to further customize bootstrap. With !default sass will only define a variable if it has not already been set.

This allows for more flexibility.

//Example1 Dress color = red
$auroras-dress-color: blue;
$auroras-dress-color: red;

//Example2 Dress color = red
$auroras-dress-color: blue !default;
$auroras-dress-color: red;

//Example3 Dress color = blue
$auroras-dress-color: blue;
$auroras-dress-color: red !default;

So Why is this important? Bootstrap is a package. Most people don't edit the Bootstrap source. NEVER UPDATE THE BOOTSTRAP SOURCE. To customize bootstrap you will add your own variable file and compile it with the bootstrap code but never touch the native bootstrap package. Bootstrap sass's page has the full skinny on how to customize and compile it in the documentations.

I don't know why less does not do this. I have not worked much with less and do not know if it has it's own built in variable management.

Example fiddle https://jsfiddle.net/siggysid/344dnnwz/