I would disagree with applying this only to explicit u-photo, I think that would result in a surprise to web authors. The simpler model is to handle "alt" for u-photo regardless of whether it is implicit or explicit.
In addition, why shouldn’t it apply to any use of u-* with an img?
E.g. "u-featured" on an img should also pick up any alt attribute.
In short, I’d rather NOT go through multiple proposal/consensus/prototype/changes to get "alt" to work properly. I’d rather we figure out how "alt" should work and change the parsing spec once to handle it.
Note the issue name "image alt text is lost during parsing" is not specific to u-photo. Let’s fix this for any use of any image (img) tags in the parsing spec.