Trying to read it in Opera i was "greeted" with "1st line should be XML declaration" error.
I tried few validators:
- Same error on http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fstackoverflow.com%2Ffeeds%2Ftag%2Fdelphi
- Probably related error on http://www.validome.org/xml/validate/?lang=en&url=https://stackoverflow.com/feeds/tag/delphi
- NETWORK_ERROR at http://www.w3schools.com/xml/xml_validator.asp (well, hopefully it does not CAUSE it :-) )