How does 'purging' unused CSS work in tools like PurgeCSS?
Jun 22, 2025 am 12:30 AMPurging unused CSS works by scanning your project files to find which CSS classes are actually used, and removing the ones that aren't. Tools like PurgeCSS help reduce CSS file sizes dramatically by eliminating styles that don’t apply to your final website or app.
How does it identify unused CSS?
The core idea is simple: these tools compare the CSS selectors in your stylesheets with what’s actually present in your HTML (or other template files), JavaScript components, etc.
- They parse through all the HTML, JSX, Vue templates, or similar files you specify.
- Then they extract every class name, ID, or tag that appears in those files.
- Finally, they check your CSS for matching selectors — anything not matched gets removed.
This doesn’t just look at class names literally; it also considers how frameworks might dynamically build class names or use special directives.
What settings affect the purging process?
Most tools give you options to control how aggressively they clean up:
-
Content paths: You tell the tool where to look for files that may contain class names — things like
./src/**/*.html
,./components/**/*.vue
, etc. - Whitelist: Sometimes a class isn’t used directly in markup but is added via JavaScript (like animations or toggles). You can whitelist those so they’re not deleted.
- Keyframes and dynamic selectors: By default, some tools may remove keyframes or attribute-based selectors unless told otherwise.
- Default safelist vs custom safelist: Some tools come with built-in exceptions (like common utility-first framework patterns), but you often need to add your own if you're doing something unique.
You’ll usually configure this in a config file or as part of your build script.
When should you be careful using purge tools?
While purging can save a lot of file size, especially when using large utility libraries like Tailwind CSS, there are cases where it might accidentally remove needed styles.
- Dynamic class names — like `"text-" color in code — won't be picked up unless you write them out explicitly or add them to the safelist.
- Third-party components — if you're loading external widgets or plugins that inject their own markup, those classes won’t show up during scanning.
- Hidden elements or conditional rendering — sometimes parts of the site only appear under certain conditions. If those aren’t included in the scan, the related styles get cut.
So while the basic idea is solid, setup matters. You want to make sure all relevant files are scanned and edge cases are accounted for.
基本上就這些.
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