CSS compression bug

In case you use CSS rules like the following

.title {
  font-size: calc(1em + 2px);
}

then the white-space surrounding the plus sign should survive the CSS compression.
Otherwise, it's a CSS syntax error and the corresponding CSS rule will be ignored.
Same with `calc` in combination with the minus sign.

In case you are still using YUI compressor, then the problem is a bug there: 
https://github.com/yui/yuicompressor/issues/330
In that case, I guess, you should replace it with a modern CSS compressor.
Thank you very much in advance.