853 is a cro-mo steel,
AFAIK. Or at least probably has a similar composition to the class of steels known as 'cro-moly.' I do not know if the exact metallurgical composition of 853 is known - it may be a closely guarded secret.
http://en.wikipedia.org/wiki/Chromoly
The advantage of 853 is that it can get stronger in the heat affected zone after welding or brazing. Builders usually use this charateristic to make a lighter frame with the same strength as a heavier frame made from an inferior material. They make lighter frames by using thinner walls than would be possible with weaker materials.
As for stiffness, most steels have very similar stiffness per unit of cross-sectional area, so a frame made from thinner walled material (like 853 usually is) actually tend to be less stiff than a frame made of thicker material...
all other things being equal*. Fortunately, most steel frames are more than stiff enough and if a slight drop in stiffness is the cost for a lighter frame it is not a big problem.
* All other things are usually
not equal, though, and, as mentioned above, build quality, fit, and geometry are more important than the actual alloy used in manufacture.