Why Zero Runtime CSS-in-JS is the Future of Web Development

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A Zero runtime CSS-in-JS is a technique of writing CSS styles in JavaScript that does not require any additional runtime or library to function. This means that the styles generated by your code are immediately available and do not need to be parsed by a separate library at runtime.

The topic of zero runtime CSS-in-JS is important because it represents a significant shift in how we think about styling web applications. In the past, developers had to choose between using traditional CSS with its global namespace and other limitations or using CSS-in-JS libraries with their added complexity and performance overhead. With zero runtime CSS-in-JS, it’s possible to get the best of both worlds: the power and flexibility of JavaScript combined with the speed and ease of use of traditional CSS.

What is Zero Runtime CSS-in-JS?

A. Definition and explanation

Zero Runtime CSS-in-JS is a type of CSS-in-JS solution where the CSS styles are generated during the build-time rather than at runtime. In other words, the styles are extracted from the JavaScript code during the build process and injected into the HTML file as regular CSS files. This means that there is no need for a separate CSS file or a runtime library to generate and manage styles.

B. Benefits compared to traditional CSS

Zero Runtime CSS-in-JS has several benefits over traditional CSS:
– Improved performance: Since the styles are generated during the build process, there is no runtime overhead for generating and managing the styles. This leads to faster page loads and improved overall performance.
– Better developer experience: Developers can write styles directly in their JavaScript code without having to switch between different files or languages. This makes it easier to maintain and refactor code.
– Scoped styles: Zero Runtime CSS-in-JS solutions often provide tools for scoping styles to specific components or modules, which helps prevent style conflicts and makes it easier to write reusable components.

C. Comparison with other types of CSS-in-JS

Zero Runtime CSS-in-JS is just one type of CSS-in-JS solution. Other types include runtime CSS-in-JS, static CSS-in-JS, and CSS modules. Here are some differences between these solutions:
– Runtime CSS-in-JS: Styles are generated at runtime using a library such as styled-components or emotion. This approach provides more flexibility but also incurs a runtime performance cost.
– Static CSS-in-JS: Styles are generated during the build process but are still outputted as JavaScript objects. This approach provides better performance than runtime CSS-in-JS but can be harder to debug.
– CSS modules: A technique for scoping CSS styles to specific components or modules by automatically generating unique class names. While this approach doesn’t use JavaScript, it can be harder to integrate with other build tools and requires a separate build step.

The Future of Web Development

Current issues with traditional CSS

Traditional CSS has some limitations that can make it difficult to manage and maintain in larger projects. One of the biggest issues with traditional CSS is the global scope of styles. When you declare a class or ID selector in your CSS file, it’s available throughout your entire application. This can lead to naming conflicts, where two selectors with the same name are used for different purposes.

Another issue with traditional CSS is its lack of encapsulation. When you use traditional CSS, it’s easy to accidentally style elements on other parts of the page unintentionally. This can be especially problematic when working with third-party components or libraries, which may have their own styling.

The importance of performance in web development

Performance is becoming increasingly important in web development. Users expect sites to load quickly and be responsive. Slow-loading sites can result in lost traffic and revenue. Additionally, Google has stated that site speed is a ranking factor in search results.

CSS can also affect website performance. Large CSS files can slow down page load times, particularly on mobile devices with slower connections.

How zero runtime CSS-in-JS addresses these issues

Zero runtime CSS-in-JS libraries like styled-components and emotion provide a solution to the issues with traditional CSS. These libraries allow you to write CSS styles in JavaScript, which provides several benefits. Firstly, this allows for more encapsulated styling, as styles are tied directly to the component they’re applied to. This eliminates naming conflicts and makes it easier to reason about the code.

Additionally, zero runtime CSS-in-JS can help improve website performance. With traditional CSS, all styles are loaded upfront, even if they’re not used on the current page. With zero runtime CSS-in-JS, styles are only loaded when they’re needed, which can reduce the overall size of the CSS file and improve page load times.

Here’s an example using styled-components:

import styled from 'styled-components';

const Button = styled.button`
  background-color: ${props => props.primary ? 'blue' : 'white'};
  color: ${props => props.primary ? 'white' : 'black'};
  font-size: 1rem;
  padding: 0.5rem 1rem;

function App() {
  return (
      <Button primary>Click me!</Button>
      <Button>No, click me!</Button>

In this example, we define a styled button component that accepts a “primary” prop to determine its background and text colors. This allows us to encapsulate the styles for the button within the component, rather than having them be global. When the component is rendered, only the necessary styles are loaded, improving performance.

How it Works

Overview of how zero runtime CSS-in-JS works

Zero runtime CSS-in-JS is a technique that allows developers to use JavaScript code to generate styles for their web applications at build time, rather than at runtime. This means that the generated styles are pre-compiled and can be optimized by the build tools.

When the web page loads, the pre-compiled styles are already available and do not require any additional JavaScript to apply them to the DOM. This can lead to faster load times and better performance compared to traditional CSS stylesheets or runtime CSS-in-JS libraries.

Implementation details

Zero runtime CSS-in-JS libraries work by using JavaScript to generate CSS styles at compile-time. The generated CSS is then injected into the HTML as a style tag or a CSS file that can be loaded independently.

To accomplish this, zero-runtime CSS-in-JS libraries often rely on build tools like Webpack or Rollup to preprocess the JavaScript files before they are served to the browser. During this preprocessing step, the JavaScript code is analyzed, and the CSS styles are generated and included in the final output.

Examples of popular libraries that use zero runtime CSS-in-JS

There are many popular zero runtime CSS-in-JS libraries available, including:

Each of these libraries has its own unique syntax and features, but they all share the common goal of generating CSS at build time to improve performance and maintainability.

The Benefits

Improved site performance

Zero Runtime CSS-in-JS can improve site performance by reducing the number of HTTP requests made by a web page. With traditional CSS, all styles are loaded in the head of the document, even if they aren’t used on the current page. This can slow down the initial load time of the page. With Zero Runtime CSS-in-JS, only the required styles for a specific component or page are loaded.

Easier to maintain and manage styles

Zero Runtime CSS-in-JS reduces the complexity of managing styles by keeping them within the same codebase as the components they belong to. This means that developers no longer need to switch between different files to make changes to both components and their styles. It also reduces the risk of conflicts between styles and makes it easier to refactor components without worrying about breaking styles.

Better developer experience

Zero Runtime CSS-in-JS provides a better developer experience by allowing developers to use JavaScript syntax to style components. This provides greater flexibility, control, and customization options. Developers can also use familiar tools, such as linters and debuggers, to work with CSS in JS.

Less code to write

Zero Runtime CSS-in-JS can reduce the amount of code needed to style components by eliminating the need to write class names and selectors. Instead, developers can use inline styles or template literals to style components. This can lead to smaller file sizes and faster load times.

Potential Drawbacks or Concerns

Learning curve for developers

Zero runtime CSS-in-JS frameworks require a shift in the way developers think about styling web applications. This can be challenging for those who are used to traditional CSS methods. However, once developers become familiar with this new approach, they will likely find it easier to manage and maintain their styles.

Potential compatibility issues with certain browsers

As with any new technology, there is always the risk of compatibility issues with certain browsers. However, most zero runtime CSS-in-JS frameworks have been rigorously tested and are designed to work across all major browsers. It is important for developers to thoroughly test their applications to ensure compatibility.

Impact on accessibility

Accessibility is a crucial aspect of web development, and it is important to ensure that all users can access and use web applications regardless of their abilities. Some experts have raised concerns that zero runtime CSS-in-JS frameworks may negatively impact accessibility by making it more difficult to separate content from presentation. However, many frameworks provide features and tools to improve accessibility, such as support for ARIA attributes and focus management. As always, developers should prioritize accessibility in their applications and thoroughly test for compliance.

CSS-in-JS is a popular approach to styling web applications, but it has some drawbacks. One of the biggest issues is the runtime performance penalty of generating and injecting styles at runtime, which can slow down initial page load times and hurt SEO. However, a new approach called Zero Runtime CSS-in-JS aims to solve these issues by moving style generation out of the critical path and into the build process.

Zero Runtime CSS-in-JS is a paradigm for styling web applications that involves generating CSS styles during the build process instead of at runtime. It uses a combination of static analysis and code transformation to extract style information from components and generate optimized CSS files that can be included in the final build.

// Example of Zero Runtime CSS-in-JS using the styled-components library

import styled from 'styled-components';

const Button = styled.button`
  background-color: ${props => props.primary ? 'blue' : 'white'};
  color: ${props => props.primary ? 'white' : 'blue'};
  font-size: 1rem;
  padding: 0.5rem 1rem;
  border-radius: 4px;

function App() {
  return (
      <Button>Default Button</Button>
      <Button primary>Primary Button</Button>

In this example, the styled-components library is used to define a reusable Button component with dynamic styles based on props. When the application is built, the styled-components babel plugin extracts the styles and generates an optimized CSS file that can be included in the final bundle. This results in faster page load times and better SEO because all styles are available immediately.

Zero Runtime CSS-in-JS offers several advantages over traditional CSS-in-JS approaches:

  • Faster page load times: By generating styles during the build process, Zero Runtime CSS-in-JS eliminates the runtime performance penalty of traditional CSS-in-JS.
  • Better SEO: Because all styles are available immediately, search engine crawlers can easily parse and index the page.
  • Simplified tooling: Zero Runtime CSS-in-JS eliminates the need for complex runtime libraries and tools like webpack, improving developer productivity and reducing maintenance costs.
  • Improved code maintainability: By removing style generation from the critical path, Zero Runtime CSS-in-JS reduces complexity and makes it easier to reason about the codebase.


Zero Runtime CSS-in-JS is a new paradigm for styling web applications that involves generating styles during the build process instead of at runtime. It offers several advantages over traditional CSS-in-JS approaches, including faster page load times, better SEO, simplified tooling, and improved code maintainability.

Zero Runtime CSS-in-JS represents a significant shift in how we approach styling web applications. As more developers adopt this paradigm, we can expect to see a reduction in the use of runtime libraries and tools like webpack, as well as improvements in page load times and SEO. It also has implications for the broader trend of “shift left” development, where more tasks are moved earlier in the development process to reduce risk and improve quality.

If you’re starting a new web development project, consider using Zero Runtime CSS-in-JS for styling. While there may be some upfront costs in terms of tooling and learning curve, the long-term benefits are well worth it. As always, be sure to evaluate your specific needs and requirements before making any decisions.

About the author / 

Mohamed Rias

I'm a programmer, photographer, and proud parent. With a passion for coding and a love of capturing life's moments through my camera lens, I'm always on the lookout for new challenges and opportunities to grow. As a dedicated parent, I understand the importance of balancing work and family, and I strive to be the best version of myself in all aspects of my life.


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