This article talks about css purging which is a technique used to optimize bundle size.  

CSS Purging is an optimization method frequently utilized in web application development.
<h2>CSS Purging</h2>
The act of deleting redundant CSS codes from your web app is commonly referred to as purging or tree shaking. When building web applications, you may discover that your CSS file is brimming with a ton of unused css. By eliminating those styles which have no usage, one can effectively decrease the size of a css bundle. This reduction in file size ultimately leads to swifter loading times for webpages.
CSS purging can be done using tools like PurgeCSS or PostCSS. The CSS files are subjected to analysis by these tools which then proceed to eliminate all styles that have no use in the HTML files.
Here are some of the benefits of using PurgeCSS:
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Smaller CSS files: PurgeCSS removes unused CSS from your website, which results in smaller CSS files. Sophisticated sites are facilitated by shorter CSS documents as they boost website productivity through accelerated loading speeds.
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Reduced bandwidth usage: PurgeCSS can reduce the bandwidth usage of your website by removing unused CSS. This is because unused CSS is not downloaded by visitors to your website.
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Here's <a href="https://purgecss.com/guides/next.html">a quick guide</a> to doing css purging in next js applications.