The calculation of the returned value is awkward in case of a percentage of lightness.
However, as this error is here since quite a long time, fixing will surely break stuff...

Sorry about the double PR... :/

Take hsl.l = 60. Increasing it's value of 50% should give as a result 90%, and 30% when decreasing it.
The decreasing part is OK, however increasing the lightness by 50% gives 80%.

The current code, when used with percentages and lightness is equivalent to:

if ( val > 0 ) {
    val = (100 - hsl[prop]) * val / 100
} else {
    val = hsl[prop] * (val / 100);
}
hsl[prop] += val;

Numerically, with values as given above (val = 50, hsl.l = 60):

if ( val > 0 ) {
    val = (100 - 60) * 50 / 100 = 20;
} else {
    val = 60 * (50 / 100) = 30; /* Or = -30 ; as if we are here, val < 0 */
}

Thus, if we want to have 90% as a result, only the else part is valid.