We investigate CSS and CSS-T quantum error-correcting codes from the point of
view of their existence, rarity, and performance. We give a lower bound on the
number of pairs of linear codes that give rise to a CSS code with good
correction capability, showing that such pairs are easy to produce with a
randomized construction. We then prove that CSS-T codes exhibit the opposite
behaviour, showing also that, under very natural assumptions, their rate and
relative distance cannot be simultaneously large. This partially answers an
open question on the feasible parameters of CSS-T codes. We conclude with a
simple construction of CSS-T codes from Hermitian curves. The paper also offers
a concise introduction to CSS and CSS-T codes from the point of view of
classical coding theory