Hi,
To my knowledge css doesn’t use a default measurement and that it ignores values without measurement (px or em), “0” is accepted because 0 is same for any measurement.
However, I was going over Eric Meyer’s Reset uses:
body { line-height: 1; }
Well if a widely accepted reset uses a value other than 0 without measurement then that must be for a reason yes? Could anyone shed some light into this?
Thanks. 
When a unit is not present after the numerical value it turns into a simple multiplier. By doing that, it is supposed to be inherited to all font-sizes (whether px or em) that may have been applied to different elements in the CSS. That is, unless you reset the line-height for any specific elements in the cascade.
http://reference.sitepoint.com/css/line-height
SP CSS Reference -
A number value is used as a multiplier in the calculation of the value used for this property, which equals the specified number multiplied by the element’s computed font size. Child elements will inherit the specified value, not the resulting used value for this property. Negative values are illegal.