The CSS validator at http://jigsaw.w3.org/css-validator/ outputs an "unknown" error if the percent sign is omitted in a linear-gradient color stop:

body {
    background: linear-gradient(100deg, white 0, black 100%);
}

Whereas this validates fine:

body {
    background: linear-gradient(100deg, white 0%, black 100%);
}

The way I read the spec it's OK to omit the unit: http://dev.w3.org/csswg/css3-images/#linear-gradients