The proper quotes usage in the CSS and thus while using the quotes property should be encoded with the ISO characters, or more precisely, ISO 10646 characters. The following list describes the entire set of possible quotes used with the mentioned property:

Quotes
CHARACTER ISO 10646 (HEX) DESCRIPTION
" 0022 quotation mark [the ASCII double quotation mark]
' 0027 apostrophe [the ASCII single quotation mark]
‹ 2039 single left-pointing angle quotation mark
› 203A single right-pointing angle quotation mark
« 00AB left-pointing double angle quotation mark
» 00BB right-pointing double angle quotation mark
‘ 2018 left single quotation mark [single high-6]
’ 2019 right single quotation mark [single high-9]
“ 201C left double quotation mark [double high-6]
” 201D right double quotation mark [double high-9]
„ 201E double low-9 quotation mark [double low-9]