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Yellow, small dogs. That's because the comma is separating two adjectives or an adjective and an adjunct noun. Comma is used to separate list items. Though it is not the only use, it is one of the uses of the punctuation.
Technically you could argue that a, handsome" can work if *a is working as an adjective, in the sense of stressing "one, singular" - so you might run across something like this (probably more likely in speech, with audible commas). But that's @DhanishthaGhosh 's rule applied to an unusual situation (where a isn't just acting as an article)
Yellow, small dogs
. That's because the comma is separating two adjectives or an adjective and an adjunct noun. Comma is used to separate list items. Though it is not the only use, it is one of the uses of the punctuation.