Understanding the @mixin Directive in CSS

CSS mix-ins offer a way to group CSS declarations that can be reused throughout the website. By using CSS mix-ins, you can make your CSS code DRY (Don't Repeat Yourself), enhance its maintainability, and reduce the number of code lines. This valuable feature is not available in native CSS, but pre-processors like Sass and Less introduce the concept of @mixin to fulfill this functionality.

The correct answer to the quiz question is the @mixin directive. The @mixin directive is used to define styles that can be reused throughout the stylesheet. It's a key feature in pre-processing languages like Sass and Less which provides the ability to group CSS instructions in a more logical way. This can be particularly useful, for example, when a group of CSS properties should be applied only under certain conditions.

Practical Example

Here's an example illustrating how a @mixin directive can be applied:

// Define a mixin
@mixin border-radius($radius) {
  -webkit-border-radius: $radius;
  -moz-border-radius: $radius;
  -ms-border-radius: $radius;
  border-radius: $radius;
}

// Use the mixin
.my-element {
  @include border-radius(10px);
}

In the above example, the @mixin directive defines a 'border-radius' mixin that applies a border radius to an element. The @include directive is then used to include the styles defined in the mixin.

When this Sass code is compiled to CSS, it would give:

.my-element {
  -webkit-border-radius: 10px;
  -moz-border-radius: 10px;
  -ms-border-radius: 10px;
  border-radius: 10px;
}

Best Practices and Additional Insights

While using @mixin, it is important to remember to keep your mixins simple and minimal. Overuse of mixins can lead to bloated and hard-to-maintenance CSS files. Thus, have self-discipline and use mixins for operations that you find yourself often doing and that increase the readability and maintainability of your stylesheets.

In addition, although pre-processors like Sass and Less provide features like @mixin, modern CSS has also introduced custom properties and @apply rule which provide local and limited way of reusing CSS declarative blocks. However, at the time of writing, the @apply rule specification is withdrawn and has very limited browser support, so @mixin in pre-processors stays a thorough solution for CSS code reusability.

In conclusion, the @mixin directive acts as a powerful tool in CSS pre-processors to group reusable CSS code, contributing significantly to cleaner, DRYer, more maintainable CSS code.