Adding a semicolon after the last declaration of a rule is not mandatory. But omitting this semicolon could lead to unexpected issues.

Noncompliant Code Example

Let's take the following piece of code as an example:

.foo {
  background-color: red;
  color: green             /* Noncompliant */
}

Then, add some properties:

.foo {
  background-color: red;
  color: green             /* Noncompliant */
  width: 30px;
}

Now, you might waste time figuring out why the width declaration isn't working. Even worse, you can even fail noticing that it's not working.

Compliant Solution

.foo {
  background-color: red;
  color: green;
  width: 30px;
}