43. The faithful should stand from the beginning of the Entrance Chant, or while the Priest approaches the altar, until the end of the Collect; for the Alleluia Chant before the Gospel; while the Gospel itself is proclaimed; during the Profession of Faith and the Universal Prayer; and from the invitation, Orate, fratres (Pray, brethren), before the Prayer over the Offerings until the end of Mass, except at the places indicated here below.
The faithful should sit, on the other hand, during the readings before the Gospel and the Responsorial Psalm and for the Homily and during the Preparation of the Gifts at the Offertory; and, if appropriate, they may sit or kneel during the period of sacred silence after Communion.
In the Dioceses of the United States of America, they should kneel beginning after the singing or recitation of the Sanctus (Holy, Holy, Holy) until after the Amen of the Eucharistic Prayer, except when prevented on occasion by ill health, or for reasons of lack of space, of the large number of people present, or for another reasonable cause. However, those who do not kneel ought to make a profound bow when the Priest genuflects after the Consecration. The faithful kneel after the Agnus Dei (Lamb of God) unless the Diocesan Bishop determines otherwise.[Sacrosanctum Concilium 40; Varietates legitimae 41]
For the sake of uniformity in gestures and bodily postures during one and the same celebration, the faithful should follow the instructions which the Deacon, a lay minister, or the Priest gives, according to what is laid down in the Missal.
I’m unaware of significant portions of the US where these practices are not followed. I suppose GIRM 43c leaves the door open a crack for standing during the Eucharistic Prayer for permitting something other than kneeling for “another reasonable cause.”
Other assorted comments on the 1975 to 2000 upgrade:
New to this century is the profound bow when the priest genuflects. Also new is that last paragraph above about following instructions.