“You arenʼt gonna need it” (YAGNI) is
a principle which arose from extreme programming (XP) that states a programmer should not add functionality until deemed
necessary. Other forms of the phrase include
“You arenʼt going to need it” (YAGTNI) and “You
ainʼt gonna need it”.
“Your Mileage May Vary” (YMMV) comes from
American car ads. For decades, these ads have been listing highway and
city mileage (yes, the US still uses miles). Invariably, the mileage
given is higher than what the typical driver will achieve, hence the
disclaimer, “your mileage may vary”.
In internet parlance, YMMV typically means that the speaker/author is
expressing a personal opinion — one that might be controversial. In
this instance, YMMV means “this works for me; your experience
might be different”. It is in this sense that we use it on Craft
Code.