SRI works on external stylesheets too. The browser rejects the CSS if the hash does not match.
Generate hashes with openssl dgst -sha384 -binary file.css | openssl base64 -A or an SRI hash generator, then prefix with sha384-.
openssl dgst -sha384 -binary file.css | openssl base64 -A
sha384-
Lines: 0 | Characters: 0