This is an excellent question. I don't know. It seems that iconographical canons are simply traditional guidelines that are not taken legalistically. They do not originate from councils. Here is, for example, how Cavarnos describes the Annunciation:
On the left side of the composition stands the archangel Gabriel, having the right arm stretched out, blessing the All-holy Virgin Mary, who is at the opposite side, and holding in his left hand a staff, a symbol of his authority as a messenger of God … Gabriel wears a tunic and an outer garment. He is shown as if he were running, having one of his legs forward, with the knee protruding under his garments. His facial expression is solemn, manly, but modest. Sometimes one of his wings is raised, the other lowered.The Virgin is shown standing before a throne or seated on it. She wears long inner and outer garments that leave exposed only her face. The front part fo the neck, and the hands. Her facial expression and posture evince modesty, prudence, and humility. Her head is inclined in the direction of the Angel. With her right hand she makes a gesture of acquiescence, while in her left hand she sometimes holds a spindle or a handkerchief. Her gaze as well as her bearing and gesture convey the .. statement from the Gospel of St. Luke: “Behold the handmaid of the Lord; be unto me according to thy word”.
In the background there are building whose elegant form enhances the beauty of the composition. Above them, at or near the middle of the top of the icon, is a representation of the heavenly vault in the form of a semicircle, from which there radiates a beam of light. The light symbolizes the grace of the All-holy Spirit. It terminates at or near the halo of the Virgin. Sometimes, certain other features ar added, if space permits—for instance a table near the Theotokos, on which is placed the Old Testament and a vase with flowers. The latter serve as a symbol of her virginity.
Here is a good icon written to that canon (the ray of light apparently was golden and nearly completely wore out):
If you google images for "annunciation icon" you will find -- even ignoring coptic or western ones -- variations in terms of artistic interpretation, as well as another type where both are standing and there is no throne:
Sometime I see the Holy Ghost (a dove) superimposed on the ray of energy; sometime, Jesus is shown in a mandorla at the top center, rather than an empty "heavenly vault". In the second image from Ustyug Jesus is shown twice, once as a baby in the womb and second time blessing from heaven.
The question becomes, is Ustyug one (both standing) non-canonical? It is certainly of ancient origin as there are coptic composition like that.
People who know icons have a feel for what is strange and out of the loop, and what is canonical: wrong color or style of Our Lady's garment, wrong facial type, wrong posture. However, they do not treat these as lawyers, rather they would say "I've never seen it done like that".