In SASS, if you use an operation to calculate a value, you then have to concatenate the unit. If you just append the unit, the resulting compiled CSS has a space between number and unit.
Ex.
font-size: ($size / $base)em;
Compiles into:
font-size:2 em;
This is an error in CSS, because 2 em
is not an accepted property value. So, you need to concatenate the number and the unit, like this:
font-size:($size / $base)+em;
Compiles into:
font-size:2em;
DETAILS
Files:
core-mixins.scss lines 8-9-60-62-64
core.scss lines 212-218-222-228-283-286-457
UPDATE 2014-05-21
Concatenating number and unit turns the value into a string. In order to just convert a unit-less number to one with a unit, just multiply it for one desired unit.
E.g.
$size: 2;
$base: 1;
($size / $base)em
compiles into 2 em
(not accepted property value)
($size / $base) + em
compiles into 2em
(string - accepted by CSS, but may cause problems in SASS)
($size / $base) * 1em
compiles into 2em
(number, em value - good both for CSS and SASS)