CSS aggregation corrupts some css rules

I have a custom css file containing a rule like this:

.field--name-field-pages:before {
    content: ', pages ';
}

Note the space after the comma and before "pages" in the value for 'content'. If i enable "CSS aggregation" in Configuration->Development->Performance, then this space will not be there in the resulting aggregated CSS on the page. But it works fine without aggregation.

Drupal 8.1.10.