CSS, height makes the border color change

Question

Sometimes CSS acts weird in chrome, the difference between the two divs are only the height parameter, but as a result: the border color is different.

body {
background: black;
}

#div1 {
    border: 1px solid white;
    height: 41px;
    width: 100px;
    transform: rotateZ(270deg);
    transform-origin: right;
}

#div2 {
    border: 1px solid white;
    height: 40px;
    width: 100px;
    transform: rotateZ(270deg);
    transform-origin: right;
}

<div id="div1">

</div>

<br><br><br><br><br><br>

<div id="div2">

</div>

PS : I am working under chrome with 100% zoom

result :

Answer 1

This is in fact an anti aliasing issue. The 1px border is in between two pixels, resulting in this grey looking color.

To visualize this I have taken a screenshot for you, zoomed in and put a grid over it:

Keep that in mind when transforming objects.