Neither. It's not measured from the descender or the baseline. Margins start from the bottom of the element (including any padding and border) regardless of what text is inside of this. You can see this yourself if you use your developer tools built into your browser (F12 or right click + inspect element).
The only affect that text has on this is the total height, which is in reality set by
line-height and the total number of lines, not the text itself.
In this image there is both a
margin-bottom of 20px on the top div and a
margin-top of 20px on the bototm div, but since margins collapse by default the margins combine to form a total of 20px space in between. If you want to disable this behavior you can, but I don't recommend it most of the time.