CSS MIME Type

About CSS stylesheet files

Browsers refuse to apply a stylesheet sent with another type when the page is in standards mode or the server sends nosniff. The classic error, “Refused to apply style because its MIME type ('text/html') is not a supported stylesheet MIME type”, nearly always means the CSS URL returned an HTML page: a 404 page or a single-page-app fallback.

File signature

CSS has no fixed signature: it is text without a magic number, so validate uploads by parsing the content rather than by checking bytes.

Serve .css with the right Content-Type

Content-Type: text/css; charset=utf-8

text/css css;

AddType text/css .css
AddCharset UTF-8 .css

@css path *.css
header @css Content-Type "text/css; charset=utf-8"

curl -sI https://example.com/file.css | grep -i '^content-type'
# Content-Type: text/css; charset=utf-8

Set the type in code

res.set('Content-Type', 'text/css; charset=utf-8');

import mimetypes

mimetypes.add_type('text/css', '.css')  # if your system doesn't know it
mimetypes.guess_type('file.css')[0]  # 'text/css'

mime.AddExtensionType(".css", "text/css; charset=utf-8")
w.Header().Set("Content-Type", "text/css; charset=utf-8")

Accept .css uploads

<input type="file" accept=".css,text/css">

The accept attribute only filters the file picker; users can still choose another file. Listing the extension as well as the type matters, because operating systems don’t always report the type you expect.

Related types

• .html (text/html): HTML
• .js (text/javascript): JavaScript
• .woff2 (font/woff2): WOFF2 font

More web

.mjs , .svg , .wasm , .webmanifest , .rss , .vtt . Look up any extension or type with the MIME type lookup.

Sources

• IANA registration: text/css
• MDN: Common media types

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Frequently asked questions