What is a Sass placeholder selector?

A Sass placeholder selector is a special type of selector that is used to define styles that will be extended by other selectors, but will not be output in the final CSS. Placeholder selectors start with a percent sign (%) and are used in conjunction with the Sass @extend rule to share a set of CSS properties across multiple selectors, while keeping the final CSS code clean and organized.

For example, let's say you have a class called .button that defines a set of styles for buttons on your website. Instead of using the .button class directly, you can use a placeholder selector to define the styles, like so:

%button {
  background-color: blue;
  color: white;
  padding: 10px;
}

.submit-button {
  @extend %button;
  font-weight: bold;
}

This will output the CSS as:

.submit-button {
  background-color: blue;
  color: white;
  padding: 10px;
  font-weight: bold;
}

As you can see, the placeholder selector %button is not output in the final CSS, and the styles are instead shared directly with the .submit-button class. This makes the CSS code more readable and easier to maintain, as you can see exactly which styles are being shared by each class.

Placeholder selectors can also be used with nested selectors to create more modular and maintainable code. For example, you can use a placeholder selector to define the styles for a specific section of your website, and then extend those styles to all the elements within that section.

%section {
  background-color: #f2f2f2;
  padding: 20px;
}

#about {
  @extend %section;
}

#about h2 {
  @extend %section;
}

In summary, a Sass placeholder selector is a special type of selector that is used to define styles that will be extended by other selectors, but will not be output in the final CSS. It is used in conjunction with the Sass @extend rule to share a set of CSS properties across multiple selectors, while keeping the final CSS code clean and organized. Placeholder selectors make it easy to create more modular and maintainable code, and can help to improve the readability of your CSS.