Warning Unquoted multi-word font-family name

Example

/* Wrong: two words parsed as two separate identifiers */
.body { font-family: Open Sans, sans-serif; }

/* Right */
.body { font-family: "Open Sans", sans-serif; }

Why it matters

CSS's font-family value is a comma-separated list where each entry is either a quoted string or an unquoted CSS identifier. An identifier can technically contain spaces if every word is a valid identifier token, which "Open" and "Sans" individually are — so font-family: Open Sans often does work in practice, which is exactly what makes it a fragile habit rather than a reliably safe one. Font names that start with a number, contain punctuation other than a hyphen, or coincide with a CSS-reserved keyword can fail to parse as an unquoted identifier at all. Quoting removes the ambiguity entirely and matches how a font name is written literally everywhere else (the OS font list, an @font-face declaration's font-family descriptor, and so on).