fake error on embed fonts

calibre editor show some "errors" about embed fonts
show error with font-family: "Charter";
Unexpected missing generic font family (font-family-no-missing-generic-family-keyword). See detailed description suggest
and here the link to which i'm referring https://stylelint.io/user-guide/rule...family-keyword)
but no error with font-family: "Charter", serif;

but on my iPad mini no difference with or without serif;
so fake error for me ... i hope the dev can add an option for future next rel to hide or ignore error like this.