Property merging

Property merging is useful if properties accept a Comma Separated Value (CSV). You will find this type of property mostly in CSS3, where borders, backgrounds, and transitions accept a CSV list. However, you will also find that the old-school font-family parameter accepts a list of font names that are separated by commas. Properties are merged by adding a plus sign (+) after their names, as shown in the following code:

.alternative-font()
{
  font-family+: Georgia,Serif;
}
.font()
{
  font-family+: Arial;
  .alternative-font;
}
body {
.font;
}

This code will get compiled into the following code:

body {
  font-family: Arial, Georgia,Serif;
}