Minification of CSS in -p mode removes -moz/-o/-ms rules

I was testing out Webpack with the -p flag and got some weird results in my CSS compared to my regular page. One of these errors was the aparent removal of CSS code.

CSS in my .css-file:

.systembar::before {
  background: #0066b3;
  background: -o-linear-gradient(left, #a2c517 10%, #009046 30%, #0082b6 50%, #151f77 75%, #db0050 100%);
  background: -moz-linear-gradient(left, #a2c517 10%, #009046 30%, #0082b6 50%, #151f77 75%, #db0050 100%);
  background: -webkit-linear-gradient(left, #a2c517 10%, #009046 30%, #0082b6 50%, #151f77 75%, #db0050 100%);
  background: -ms-linear-gradient(left, #a2c517 10%, #009046 30%, #0082b6 50%, #151f77 75%, #db0050 100%);
  background: -webkit-gradient(linear, left top, right top, color-stop(0.1, #a2c517), color-stop(0.3, #009046), color-stop(0.5, #0082b6), color-stop(0.75, #151f77), color-stop(1, #db0050));
  background: linear-gradient(left, #a2c517 10%, #009046 30%, #0082b6 50%, #151f77 75%, #db0050 100%);
}

After inspecting the ::before object in Firefox:

{
    background: #0066b3;
    background: -webkit-linear-gradient(left,#a2c517 10%,#009046 30%,#0082b6 50%,#151f77 75%,#db0050);
    background: -webkit-gradient(linear,left top,right top,color-stop(.1,#a2c517),color-stop(.3,#009046),color-stop(.5,#0082b6),color-stop(.75,#151f77),color-stop(1,#db0050));
    background: linear-gradient(left,#a2c517 10%,#009046 30%,#0082b6 50%,#151f77 75%,#db0050);
}

This is of course not matched to Firefox, and the background becomes blue instead of gradient

It seems that the minification done by -p removed the Firefox/Opera/IE-specific lines, and kept only the Wekbit parts. Why is this done, and how can I prevent this bug?

[SimenB]

adding -autoprefixer as an option to css-loader fixes this in our case. We use autoprefixer ourselves to manually add support for stuff for quite old IEs and Chromes, which cssnano removed

[robinsving]

Moved issue here though it was already answered by @SimenB in this issue. Closing.