What does “a” stand for in font: 0/0 a;

Question

I was referring a video tutorial where the designer used font: 0/0 a; for image replacement, so I get that 0 is the font-size, another

Answer 1

http://nicolasgallagher.com/another-css-image-replacement-technique/

font:0/0 a – a shorthand property that zeros out the font size and line-height. The a value acts as a very short font-family (an idea taken from the BEM implementation of this method). The CSS validator complains that using 0/0 in the shorthand font property is not valid, but every browser accepts it and this appears to be an error in the validator. Using font:0px/0 a passes validation but it displayed as font:0/0 a in the code that the validator flags as valid.