CDN should support ETAGS just like they support last modification date
When performing a request with 'If-Modified-Since' header, CDN correctly respond 304 if data is not modified. But they ignore equivalent ETAGS directive: IF-None-Match and always return OK 200. They should consider it. Just as described in RFC 7234.
This is a limitation just with Azure CDN from Akamai. With Azure CDN from Verizon ETag support is enabled by default. Supporting ETags is an optional and not a mandatory header per HTTP RFC. We are working long term to mitigate / remove differences in caching behavior between Verizon and Akamai.