The following CSS in a plugins styles.css (not tested with themes) will not survive minification after purging caches:

left: calc(50% - 15em / 2);
left: calc(50% - 30px / 2);
left: calc(50% - 50% / 2);

There is a workaround, which is to do the last division calculation manually:

left: calc(50% - 7.5em);
left: calc(50% - 15px);
left: calc(50% - 25%);

However, sometimes it is nice to use calc for code readability- e.g.

width:15%;
left: calc(50% - 15% / 2);

Because I have the width at 15% and I am using the same value but dividing it by 2 in the next line, it is more obvious what I am trying to achieve as opposed to just writing 7.5%

Steps to replicate:

In any plugin with a styles.css file add:

.bugtest {

    left: calc(50% - 15em / 2);

    left: calc(50% - 30px / 2);

    left: calc(50% - 50% / 2);

    top: calc(50% - 7.5em);

    top: calc(50% - 15px);

    top: calc(50% - 25%);

}

purge caches

In network tab inspect css for styles.php / all - e.g:

http://gthomas2moodle.dev/theme/styles.php/clean/1481275721/all

Notice that the css for the .bugtest class contains the "top" attributes but not the "left" attributes.