As much as I appreciate Longhorn’s support, I really would appreciate it they did not include Atom 0.3 support. By the time Longhorn comes out, I have every intent to make Atom 0.3 feeds as rare as Atom 0.2 feeds are now; which is to say, practically non-existent.
For now, however, Atom 0.3 feeds continue to validate. I don’t believe I have broken anything (and have 600 test cases backing me up on this), but I will say this: while I do have checks in place ensuring that no Atom 0.3 unique elements are contained in Atom 1.0 feeds, I haven’t been so careful about ensuring the reverse checks are in place. This means that the Feed Validator has become slightly more liberal in interpreting Atom 0.3 compliance.
In early August, I plan to have the Feed Validator start issuing
warnings whenever it encounters an Atom 0.3 feed. It already
is issuing warnings on the use of Atom’s
application/atom+xml mime type by anything other than
an Atom 1.0 feed.
Possibly as early as October, and certainly no later than the end of the year, these warnings will be converted over to errors.