The WebCrypto spec says the alg value of ES521 means ESCDSA with P-521 [1]

Chromium however recognizes ES512 rather than ES521 [2].
This causes JWK import of ECDSA keys to not be spec compliant for ES521 - an issue uncovered by Jim Schaad

That said, there is something strange with the spelling of these algorithms as given by WebCrypto, since in the JWA spec definitions are given for these algs:

 * ES256 -- ECDSA using P-256 and SHA-256
 * ES384 -- ECDSA using P-384 and SHA-384
 * ES512 -- ECDSA using P-521 and SHA-512

The name ES256/ES384 here have been re-purposed by WebCrypto to mean something different (binding to curve, not hash + curve). ES521 is unambiguous though since it is only defined by WebCrypto, so does not offer any challenge to supporting.

Will need to dig through the spec history and discussions to see if this issue was alredy raised and agreed upon...

[1] https://dvcs.w3.org/hg/webcrypto-api/raw-file/tip/spec/Overview.html
[2] https://code.google.com/p/chromium/codesearch#chromium/src/components/webcrypto/algorithms/ecdsa.cc&sq=package:chromium&type=cs&l=167
[3] https://tools.ietf.org/html/rfc7518