Grouping Classes in CSS

Let's say we have several classes that paint elements in different colors:

css.xxx {
  color: green;
}
.yyy {
  color: red;
}

Let's say we want to make it so that these classes, in addition to color, also set the font size:

css.xxx {
  color: green;
  font-size: 30px;
}
.yyy {
  color: red;
  font-size: 30px;
}

Since both classes set the same font size, let's use selector grouping like this:

css.xxx {
  color: green;
}
.yyy {
  color: red;
}

.xxx, .yyy {
  font-size: 30px;
}

css.eee {
  font-size: 20px;
  line-height: 1.5;
  font-family: Arial;
}
.zzz {
  font-size: 30px;
  line-height: 1.5;
  font-family: Arial;
}

css.eee {
  font-size: 20px;
  text-align: center;
  font-family: Arial;
}
.zzz {
  font-size: 30px;
  text-align: center;
  font-family: Arial;
}
.ggg {
  font-size: 35px;
  text-align: center;
  font-family: Arial;
}