Writing CSS is really simple and straightforward, so why is there a need for principles and best-practices while writing CSS?

As the project scope increases and as the number of people working on the project increases, the problems become more and more apparent and can cause serious issues down the line. Fixing issues may become harder, duplicated code, complex override chains and use of !important, leftover / unused code (removed elements or features), code that is hard to read, etc.

Writing CSS at a professional level will make the CSS code more maintainable, extensible, understandable and cleaner. We're going to look at the five simple and very effective principles that will take your CSS to the next level.

Naming principle

"There are only two hard things in Computer Science: cache invalidation and naming things." -- Phil Karlton

Properly naming and structuring your CSS selectors is the first step to making your CSS more readable, structured and cleaner. Establishing rules and constraints in your naming convention makes your code standardized, robust and easier to understand.

This is why concepts like BEM (Block-Element-Modifier), SMACSS (Scalable and Modular Architecture for CSS) and OOCSS (Object Oriented CSS) are popular among many frontend developers.

Low specificity principle

Overriding CSS properties is very useful, but things can go out of hand pretty quickly on more complex projects. Overriding chains can get really long and complex, you might be forced to use !important to solve the specificity issue and you could get really easily lost when debugging or adding new features.

/* Low-specificity selector */
.card {}

/* High-specificity selectors */
.card .title {}

.blog-list .card img {}

.blog-list .card.featured .title {}

#js-blog-list .blog-list .card img {}

Browser and specificity

One of the benefits of following the low specificity principle is performance. Browsers parse the CSS from right to left.

Let's take a look at the following example:

.blog-list .card img {}

Browsers parse the selector like this:

  1. Find all img elements on the page
  2. Keep selected elements that are the descendants of .card class
  3. Keep selected elements that are the descendant of .blog-list class

You can see how high-specificity selectors impact performance, especially when we need to globally select generic elements like div, img, li, etc.

Using the same level of specificity

By using low specificity CSS class selectors in combination with BEM or one of the other naming principles mentioned in the previous section, we can create a performant, flexible and understandable code.

Why use CSS classes? We want to keep the same level of specificity, stay flexible and be able to target multiple elements. Element selectors and id selectors do not offer the flexibility that we need.

Let's rewrite our previous example using BEM and keeping specificity low.

/* Low-specificity selector */
.card {}

/* Fixed high-specificity selectors */
.card__title {}

.blogList__image {}

.blogList__title--featured {}

.blogList__img--special {}

You can see how these selectors are simple, understandable and can be easily overridden and extended if needed. And by keeping them low-level (a single class), we are guaranteed optimal performance and flexibility.

DRY Principle

DRY (Don't repeat yourself) principle can be also applied to CSS. Duplicated code in CSS can cause code bloat, unnecessary overrides, reduce maintainability, etc. This issue can be fixed by structuring the code appropriately and having high-quality documentation.

Storybook is a great free tool that enables you to create an overview of available frontend components and write high-quality documentation.

/* Without DRY Princple */
.warningStatus {
  padding: 0.5rem;
  font-weight: bold;
  color: #eba834;
}

.errorStatus {
  padding: 0.5rem;
  font-weight: bold;
  color: #eb3d34;
}

.form-errorStatus {
  padding: 0.5rem 0 0 0;
  font-weight: bold;
  color: #eb3d34;
}

Let's refactor the code so it follows the DRY principle.

/* With DRY Principle */
.status {
  padding: 0.5rem;
  font-weight: bold;
}

.status--warning {
  color: #eba834;
}

.status--error {
  color: #eb3d34;
}

.form__status {
  padding: 0.5rem 0 0 0;
}

Single responsibility principle

By using the single responsibility principle in our CSS, we can ensure that our CSS classes are easily extended and overriden. Let's take a look at the following example.

.button {
  padding: 1rem 2rem;
  font-size: 2rem;
  border-radius: 0.2rem;
  background-color: #eb4934;
  color: #fff;
  font-weight: bold;
}

.button--secondary {
  border-radius: 0;
  font-size: 1rem;
  background-color: #888;
}

We can see that if we want to extend .button class with .button--secondary, we are doing lots of overrides to achieve what we need, when we only want to apply a different background color and keep the default styles.

The problem is that our .button class is having several roles:

  • Sets layout (padding)
  • Sets typography (font-size,font-weight)
  • Sets presentation (color,background-color, border-radius)

this makes our CSS classes very hard to extend and combine with other CSS classes. By keeping this in mind, let's use BEM and OOCSS to improve our CSS.

/* Shared styles */
.button {
  padding: 1rem 2rem;
  font-weight: bold;
  color: #fff;
}

/* Style extensions */
.button--radialBorder {
  border-radius: 0.2rem;
}

.button--large {
  font-size: 2rem;
}

.button--primary{
  background-color: #eb4934;
}

.button--secondary {
  background-color: #888;
}

We have broken down our button styles into several classes that can be used to extend the base button class. We can optionally apply the modifiers and add new ones as the design changes or new elements are being added.

Open/Close principle

software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

We've already used the open/close principle in the previous examples. All new features and options need to be added by extension. Let's take a look at this example.


.card {
  padding: 1rem;
}

.blog-list .card {
  padding: 0.5em 1rem;
}

The .blog-list .card selector has few potential issues:

  • Some styles can be applied only if the .card element is a child of .blog-list element.
  • Styles are forcibly applied to the .card element if placed inside the .blog-list element, which can produce unexpected results and unecessary overrides.

Let's rewrite the previous example:

.card {
  padding: 1rem;
}

.blogList__card {
  padding: 0.5em 1rem;
}

We've fixed the issue by having a single class selector. With this selector, we can avoid unexpected effects and there are no conditional nested styles.

Conclusion

We've seen how by applying these few simple principles we have significantly improved the way we write CSS:

  • Standardized naming and structure, and improved readability by using BEM, OCSS, etc.
  • Improved performance and structure by using low-specificity selectors.
  • Reduced code bloat and improved code quality with DRY principle
  • Flexibility and maintainability by using open/close principle
  • etc.

Thank you for taking the time to read this post. Feel free to share your thoughts about this topic in the comments or drop us an email at hello@prototyp.digital.