.CSS file extension
To open .CSS files on Windows, right-click the .css file → Open with → choose a text/code editor (for example, Visual Studio Code).
To open a .css file, use a text editor or a code editor (for example, Visual Studio Code). A .css file is plain text that contains CSS rules used by web browsers to style HTML documents.
Last updated: April 30, 2026 · Reviewed by Julian Stricker
Open on your device
Choose your operating system for a dedicated step-by-step opening guide.
How to open .CSS files
Use these platform-specific instructions to open .CSS files safely.
Windows
- Right-click the .css file → Open with → choose a text/code editor (for example, Visual Studio Code).
- If you want to see the effect on a web page, open the related HTML file in a browser; the browser applies the .css when the HTML links to it.
Mac
- Control-click the .css file → Open With → choose a text/code editor (for example, Visual Studio Code).
- To test styling, open the associated HTML file in a web browser so it loads the stylesheet.
Linux
- Right-click the .css file in your file manager → Open With → choose a text editor or code editor.
- If file associations are wrong, set the default application for the MIME type text/css in your desktop environment.
iOS
- In the Files app, tap the .css file to preview it as text; if you need editing features, share/open it in a text/code editor app that supports plain text.
Android
- Open the .css file from your file manager; if it doesn’t show as text, choose an app that can open plain-text files (a text/code editor app).
Security notes
- A .css file is plain text and does not contain macros like office documents, but treat it as untrusted input: malformed CSS can still trigger bugs in CSS parsers (for example, in browsers or other tools).
- Be cautious when a .css file comes from an untrusted site or package: CSS can reference external resources (such as fonts or images) that may cause network requests when a page is loaded.
- If you are configuring a server, serve .css as text/css; incorrect MIME types can cause unexpected handling by clients and may prevent styles from loading.
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Can't open this file?
These are the most common causes and fixes when .CSS files fail to open.
Common reasons
- Browser does not apply the CSS (styles not showing)
- The .css file opens in the wrong app
- Strange characters or broken formatting when opened
Fix steps
- Check that your HTML references the correct file name and path for the .css file.
- Verify your web server is serving the file with the MIME type text/css.
OS-specific troubleshooting
What is a .CSS file?
A .css file contains Cascading Style Sheets, a stylesheet language for describing the presentation (formatting, layout, fonts, colors) of documents such as HTML. It is typically served on the web with the media type (MIME type) text/css. Because it is plain text, it can be viewed and edited without special conversion tools.
Background
CSS is a stylesheet language used to separate a website’s content (HTML) from its presentation (styles). A .css file commonly includes selectors and declarations (properties and values) that tell a browser how to display elements on a page.
On the web, servers and browsers rely on the registered media type text/css for CSS resources. If a server sends the wrong MIME type, browsers may refuse to apply the stylesheet or may treat it as plain text.
In practice, .css files are used in nearly every website and web app, and they are also used by many tools that generate or process web assets. Since CSS is text-based, collaboration tools (diff/merge) work well with it, and developers often manage it with version control.
Common MIME types: text/css
Further reading
Authoritative resources for more details on the .CSS format.
Common .CSS issues
Browser does not apply the CSS (styles not showing)
This often happens when the HTML file is not linking to the correct stylesheet path, or when the server sends the wrong MIME type instead of text/css.
- Check that your HTML references the correct file name and path for the .css file.
- Verify your web server is serving the file with the MIME type text/css.
The .css file opens in the wrong app
Your system’s file association or MIME type mapping may be set to an unexpected application.
- Use “Open with” and select a text/code editor.
- Change the default app association for .css (or for the MIME type text/css on Linux desktops) so future opens use your editor.
Strange characters or broken formatting when opened
The file may be saved with an unexpected text encoding or may be corrupted/incomplete.
- Open the file in a code editor that lets you choose the file encoding and try a different encoding if needed.
- Re-download or re-copy the file from the original source if it appears truncated or corrupted.
FAQ
What is a .css file used for?
It contains Cascading Style Sheets rules that control the presentation (layout, colors, fonts, spacing) of web content such as HTML.
What MIME type should .css be served as?
text/css.
Can I open a .css file in a web browser?
You can view it as text in a browser, but CSS is usually meant to be linked from an HTML document so the browser can apply it as styling.
Can I convert a file to .css by renaming the extension?
No. Renaming only changes the filename. To create a valid .css file, you must generate or write actual CSS content.
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