parent-fit in fontFamily is ignored

https://github.com/aFarkas/lazysizes/blob/1523a4ff4579e170355c7607f445689b43229caf/plugins/parent-fit/ls.parent-fit.js#L65

I like to set the parent-fit in CSS, but the content is always "normal" and ignores the setting in the fontFamily. If I use content to declare, it will run. I think Chrome set a default content in getComputedStyle(element) to "normal"