Correct handling of `<family-name>` in font-family

[yisibl]

Input

.foo {
  font-family: 'sans-serif', 'default';
}

Output

.foo {
  font-family: sans-serif, default;
}

Expected

.foo {
  font-family: 'sans-serif', 'default';
}

font-family: sans-serif means that a generic sans-serif font family will be used, while font-family: 'sans-serif' (with quotes) refers to an actual font that goes by the name of sans-serif.

CSS Spec

Note: this means that if you really have a font whose name is the same as one of the names, or the common-keywords|CSS-wide keyword values, it must be quoted.

To illustrate this, the following unusual font family names are valid because they are quoted:
font-family: "sans-serif", sans-serif;
font-family: "default", sans-serif;
font-family: "initial", sans-serif;
font-family: "inherit", sans-serif;

The full CSS-wide keyword(common-keywords) and <generic-family> in the current specification include:

CSS-wide keyword

https://www.w3.org/TR/css-values-4/#common-keywords

"initial"
"inherit"
"unset"

<generic-family>

https://www.w3.org/TR/css-fonts-4/#generic-font-families

"serif"
"sans-serif"
"cursive"
"fantasy"
"monospace"
"system-ui"
"emoji"
"math"
"fangsong"
"ui-serif"
"ui-sans-serif"
"ui-monospace"
"ui-rounded"

Here's a lint tool: https://mothereff.in/font-family

[devongovett]

Fixed by facbffd

[yisibl]

There are more case test failures:

https://developer.mozilla.org/en-US/docs/Web/CSS/font-family#valid_family_names

Input

.foo {
  font-family: "Goudy Bookletter 1911", sans-serif;
  font-family: "test@foo", sans-serif;
  font-family: "#POUND", sans-serif;
  font-family: "Hawaii 5-0", sans-serif;  
}

.bar {
  font-family: "Red/Black", sans-serif;
}

Output

.foo{font-family:"Hawaii 5-0",sans-serif}
.bar{font-family:Red\/Black,sans-serif}

Expected

.foo{font-family:"Goudy Bookletter 1911",sans-serif;font-family:"test@foo",sans-serif;font-family:"#POUND",sans-serif;font-family:"Hawaii 5-0",sans-serif}
.bar{font-family:"Red/Black",sans-serif}

[devongovett]

What's invalid in that output? Looks like the identifier is properly escaped?

[yisibl]

@devongovett I added the expected results.