How much of Mass can I miss? You know, and it still counts?

Second only to questions on annulments, the above question—How much of Mass can I miss and it still counts for my obligation?— is probably the single the most common canonical question lay people ask.

Catholics have, I think, a right to have such questions answered without others looking down their noses at their supposed legalisms or getting a lecture about their alleged lack of piety (“If you really loved Jesus you would not think of it as an obligation” etc., etc.). So address it here I will try. By the way, my observations are that those asking such questions are more likely to “over-satisfy” their duties as Catholics than to skip out on them, but maybe that’s just my prejudices at work. Anyway.

Various answers to this Mass attendance question have been offered over the decades, a la: if you’re there for the first reading, or arrive by the Gospel…