Custom CSS Editor Doesn't Like Flexbox

Regarding the Custom CSS option, if you try to save any flexbox related properties in the CSS editor, they don’t save, most get stripped out. For example, entering this:

/* Flexbox container */
#content {
   display: -webkit-flex;
   display: flex;
   -webkit-flex-direction: column;
   flex-direction: column;
   -webkit-align-content: stretch;
   align-content: stretch;
}

article {
   order: 1;
}
#comments {
   order: 2;
}
#jp-relatedposts {
   order: 3;
}
#nav-below {
   order: 4;
}

Turns into this once the stylesheet is saved:

/* Flexbox container */
#content {
   display: -webkit-flex;
   display: flex;
}

article {
}

#comments {
}

#jp-relatedposts {
}

#nav-below {
}

Big pain in the butt. Why doesn’t the editor just allow you to enter whatever properties you want?

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