Subresource Integrity (SRI) is a way to secure external dependencies of web pages. From Mozilla’s technical page:
You use the Subresource Integrity feature by specifying a base64-encoded cryptographic hash of a resource (file) you’re telling the browser to fetch, in the value of the integrity attribute of any
An integrity value begins with at least one string, with each string including a prefix indicating a particular hash algorithm (currently the allowed prefixes are
sha512), followed by a dash, and ending with the actual base64-encoded hash.
An integrity value may contain multiple hashes separated by whitespace. A resource will be loaded if it matches one of those hashes.
Supported in Chrome 45+, Firefox 43+, recently (2017-05-09) in Safari but “UNDER CONSIDERATION” in Edge since August 06, 2014.
More on this subject here.
Online scanner: sritest.io – SRI Hash Website Scanner