CSS is the backbone of web design, informing how websites look, feel, and function for the user. However, when websites are built with unoptimized CSS, this can slow down their performance, leading to lower user engagement and lower SEO rankings. Improving a website's speed and enhancing the user experience are crucial factors in optimising CSS, which entails quality code. To achieve this, developers need to master advanced CSS techniques that will help them write cleaner scripts and improve the overall performance of their site. This blog will delve into some of these techniques. Minimise and Compress CSS Why Reducing CSS File Size Matters: The amount of content in your CSS files directly affects how long your site loads. The more content, the longer the browser downloads and renders the file, especially for users on slow networks. Smaller CSS files will improve your load time, leading to a better user experience and improved SEO. Minifying CSS: Minification removes all the unnecessary characters when executing your CSS (such as spaces, line breaks, and comments) without impacting the functionality. It makes your CSS file smaller and, therefore, faster to load. CSSNano, CleanCSS, and UglifyCSS are some of the most popular automated minification tools for your CSS files. They will ensure your code is lean, mean, and not bloated while still being readable and fully functional during development. Compressing CSS Files: Compression is another way of shrinking the size of your CSS files before serving them to the client. You can compress your files on the server using Gzip or Brotli, meaning they get delivered as quickly as possible. Most browsers will automatically decompress the files for the user, meaning they are served as quickly as possible without sacrificing your styling. Practical Tips for File Organization: Once you've done that, organise your CSS to ensure maximum performance. Suppose you combine several CSS files into a single CSS file. In that case, you'll reduce the number of HTTP requests the browser makes, and fewer HTTP requests mean faster pages. So combine them when you can: instead of linking to three stylesheets, link to just one. Use CSS Preprocessors (Sass/LESS) Efficiently Benefits of Using CSS Preprocessors: You can write code that is easier to maintain and reuse with CSS preprocessors such as Sass and LESS. You can use variables, mixins, and functions to help you manage your large style sheets. You can develop more scalable styles, reduce repetition, and follow the DRY (Don't Repeat Yourself) principles, which results in easier-to-manage and modify code. Avoid Over-Nesting: Nesting comes from using preprocessors, which is one of the reasons why we find it so readable. However, deep nesting creates overly specific selectors, which can slow rendering. Browsers spend more time matching elements when you use highly specific selectors. Keep nesting shallow, and you'll avoid the performance issue while keeping your code clear. Limiting it to three levels deep is the guideline to follow. Optimise Mixers and Extend: Although mixins are great for code reuse, you can end up with bulky CSS output if you use mixins that contain many styles, so it's best to use them sparingly. Also, be careful when using @extend to avoid accidentally duplicating styles across your project. Use the mixins and @extend features, and keep your CSS as lean as possible. Leveraging Partials and Imports Effectively: Keeping large CSS files small by splitting them into partials makes the code easier to maintain. Preprocessors have us break our CSS into logical chunks and import the necessary partials. However, ensure that you aren't increasing the number of files too much, as this can hurt performance. After compiling, a handful of small files should get bundled into a single stylesheet to decrease the number of HTTP requests that must be made while maintaining the modular aspect. Avoid CSS Bloat and Unused Styles. What is CSS Bloat? This is known as CSS bloat and can occur when you add libraries or frameworks that contain a lot of styles you never use or when a redesign leaves unused styles behind. Remove Unused CSS: Tools such as PurifyCSS, UnCSS, and PurgeCSS will scan your project and remove the styles of unneeded CSS selectors. You can get rid of useless styles and shrink your CSS files. This is a great idea, especially when you are using a large framework or have iterated your website many times and have a lot of layers of styles. Your CSS should contain only the styles that are used on the page. Use Utility-first Frameworks Cautiously: Though utility-first CSS frameworks such as TailwindCSS come with a powerful set of reusable classes, they can still lead to bloat if used incorrectly. This problem can be solved by purging unused classes during the build process. TailwindCSS, for instance, has built-in helpers to strip out unused utility classes and can make the final CSS file as small as just a few kilobytes while maintaining flexibility and ease of use. Load CSS Conditionally: With media queries or lazy loading, you can rather choose to load CSS conditionally depending on the type of device and the user's screen size. So, you load the styles required for the user's device, not those for all screen sizes upfront, reducing the startup time when you first visit the site. Optimise CSS Selectors Why CSS Selectors Impact Performance: Browsers read and parse CSS selectors from right to left: the longer a selector, the longer it takes to match the element (the deeper the descendant selector, the longer the universal selector and any inefficient combinations). This can slow down rendering times, especially on larger pages. But the more efficient your selectors are, the faster your pages will render. Writing efficient selectors not only makes your pages more performant, but they also make your CSS simpler. Best Practices for Writing Efficient Selectors: The longer and more complex the selector, the more the browser has to check for matches (the specifics of how it does this are complex, but it is similar to the binary search algorithm used to search through long lists). Therefore, make selectors as short and simple as possible, using classes and IDs. For example, content div > span is better than container.content div > span. Avoid the universal selector (*) because it makes the browser apply a rule to every element on the page, whether it needs it or not. Conversely, avoid excessively specific selectors: combinations of class names with descendant or sibling combinators are often too specific, slowing down the matching process. Use classes and IDs as often as possible to keep your CSS fast and readable. Optimise for Minimal Reflows and Repaints: Inefficient CSS selectors can cause unnecessary reflows and repaints—the moment when the browser renders something on the screen. Reflows happen when the layout of the page changes because of size, position, or structure changes. Repaints follow when the appearance of something changes, but not the layout. Avoid selecting elements that have a nth-child() or a long descendant chain because it potentially costs a lot to recalculate everything. Minimise how much the document flow changes, and limit the use of complex or dynamic selectors to minimise the things that have to be recalculated. Reduce Render-blocking CSS What is Render-blocking CSS? Render-blocking CSS means stylesheets that prevent a web page from rendering until they are downloaded and parsed. Suppose CSS is placed in the head of the document without being optimised. In that case, the browser must wait for these files to load before rendering any content. This negatively impacts perceived performance, as slower load times frustrate users. Using Media Attributes: For example, it's common to use a link tag's media attribute for conditional CSS loading. You can use the media attribute to, for example, indicate that a stylesheet applies only to a certain screen size or a certain viewport or device. This helps to skip some stylesheets until they're needed. You can mark the non-critical stylesheets with media types (such as media=‘print' or media=‘screen and min-width: 600px)') to allow browsers to skip styles that don't apply, which lets them render critical content first. This way, you won't have to wait for non-critical styles to load before the visible part of the page can render. Inline Critical CSS: Critical CSS is the smallest CSS needed to style the above-the-fold content (what you see before scrolling). Inlining the critical CSS into the HTML will allow the browser to render the page faster because it won't have to fetch an external file for initial rendering. Not critical styles can be deferred and loaded after the above-the-fold content has been rendered. Tools such as Critical CSS can automatically extract and inline the critical styles and allow for faster loading in the optimal order for initial rendering. Asynchronous Loading of CSS: Yet another method of decreasing render-blocking is to load non-essential CSS outside the critical rendering path. By using the rel=‘preload' or rel=‘prefetch’ attribute, the browser is told to fetch stylesheets in a non-blocking way. Preload tells the browser to load the resource early but not block the rendering of the resource, and prefetch fetches the resource for future navigation. These attributes allow the browser to prioritise critical CSS. At the same time, other stylesheets are fetched in the background without blocking the rendering of the page. Leverage CSS Variables for Better Performance What are CSS Variables? CSS variables—more accurately, custom properties—are a way to store values (e.g., colours, fonts, spacings) and reuse them later in your CSS. They facilitate dynamic styling by enabling you to define a single CSS class that can be updated or overridden consistently across large projects without repeating code. For example: root { --primary-colour: #3498db; --font-size: 16px; } CSS variables can then be referenced throughout your stylesheet: body { colour: var(--primary-colour); font-size: var(--font-size); } Performance Benefits of CSS Variables: Using CSS variables to do this reduces your code's redundancy, which in turn improves performance. This is because when you define a value in one location and then use it again in another, you add the same code twice (redundancy). In large-scale projects, this makes your CSS harder to maintain and update—you'd need to either find every single instance of a redundant value and update them all or hope that the browser does a good job doing it for you when it is forced to recalculate your styles. Using a variable means you define the value only once, improving maintainability and browser performance. The browser doesn't need to recalculate your styles if the value is available everywhere once the variable is set. Use CSS Variables in the Theme: In particular, CSS variables are useful to design dynamic themes because you can define a set of different variables for some themes (e.g., light and dark mode) and switch between them without any additional CSS. body. light-theme { --background-colour: #ffffff; --text-colour: #000000; } body. dark-theme { --background-colour: #000000; --text-colour: #ffffff; } Shifting themes is as easy as toggling on a class on the body element without loading an entirely different stylesheet. Lazy Load CSS for Non-critical Elements What is Lazy Loading? Lazy loading is a strategy to defer loading non-critical resources—such as images, scripts, or CSS—from a page until needed. For CSS, this means that stylesheets for elements not immediately visible on screen—say, an image offscreen to the right or content below the fold—are loaded after the critical styles, reducing the time for the initial load. Practical Techniques for Lazy Loading CSS: For non-critical CSS, for example, you can lazy load it using JavaScript to load the styles asynchronously after the page has been rendered. As a result, only essential styles are loaded initially, reducing both load times and the number of render-blocking styles. Loading CSS asynchronously: <link rel="preload" href="styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'"> Another way is to load CSS with the media attribute and change it to all after the page has loaded: <link rel="stylesheet" href="non-critical.css" media="print" onload="this.media='all'"> This lets the browser paint that element without applying the non-critical CSS before the page has loaded and then apply that non-critical CSS after the rest of the page has been printed. When to Use Lazy Loading for CSS: Lazy loading, especially for large libraries (animation or UI frameworks, for example), is effective; you might not need them to arrive on the page immediately. Deferring styles associated with offscreen animation and third-party tools, for example, can drastically improve first-load performance. Apply lazy loading to styles that affect elements that aren't seen yet or that won't affect the user experience until later interactions. Optimise CSS Animations and Transitions Performance Impact of Animations: Although CSS animations and transitions can improve usability, they can also degrade performance, especially on slower mobile devices or browsers. Underperformance CSS animation can lead to lag, jank, or frame drops, giving users a suboptimal experience. Use Hardware-accelerated Animations: Not all CSS properties are equal. Animations on top, left, and width, for example, will trigger layout recalculations and reflows and, therefore, are not ideal in terms of performance. When possible, focus on animating the GPU-based properties, especially transform and opacity. Animating these properties will trigger hardware acceleration, and therefore, the animations will run uninterrupted because the entire layout doesn't need to be recalculated. For example, you will get better performance using transform: translate() instead of left for your sliding animation: .element { transform: translate(100px); transition: transform 0.5s ease-in-out; } Use the Will-change Property Wisely: The benefit is that it tells the browser which elements may change, allowing it to prepare in advance to optimise the animations. However, if will-change is used too frequently, it can cause performance issues due to the memory usage required. It should be applied only to elements that will see a drastic change in state and removed when no longer needed. .element { will-change: transform; } Throttle Animation Performance: Finally, to reduce the frequency of updates, use requestAnimationFrame (such as window.requestAnimationFrame, which is supported in most modern browsers) to ensure animations run at the native refresh rate. This prevents the animation from hogging the CPU or GPU, especially in CPU-heavy animations. Such an approach also automatically reduces processing overhead by running the animation updates closely in sync with the browser's rendering cycle. Conclusion Optimising CSS is vital when it comes to improving a website's loading times, user experience, and search engine rankings since techniques such as minifying, lazy loading, and optimization of animations can significantly improve performance. Developers should always optimise their CSS workflow for performance, especially since websites will only become more complex. Learn to apply these techniques today and look into tools to help automate CSS optimization so your code remains as performant as possible. Learn more about Advanced Tips for Efficient Coding Practices→