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http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fwww.whatwg.org%2Fstyle%2Fspecification&profile=css3&usermedium=all&warning=1&vextwarning=&lang=en Family names containing whitespace should be quoted. If quoting is omitted, any whitespace characters before and after the name are ignored and any sequence of whitespace characters inside the name is converted to a single space. This doesn't match current CSS specs http://dev.w3.org/csswg/css3-fonts/#propdef-font-family http://www.w3.org/TR/CSS21/fonts.html#font-family-prop In practice I think it is pointless to warn about whitespace in unquoted font family names, since they are harmless. What is problematic is unquoted family names that start with a digit or contains punctuation, and that applies even if the name contains *no* whitespace. So either you should warn about all unquoted identifiers and say that names that start with a digit or contains punctuation will break, or don't warn about valid unquoted identifiers at all.
The warning that is currently displayed only makes sense in extreme edge cases, like when the font family name actually contains two or more consecutive space characters, e.g. `Foo Bar`. In that case, you’d need to use quotes: `font-family: 'Foo Bar'`. If you’d use `font-family: Foo Bar` (without the quotes) instead, it would get interpreted as `font-family: 'Foo Bar'`, with only a single space, which wouldn’t match the desired font family name.