Do I need to wrap quotes around font family names in CSS?

I remember hearing a long time ago that it was considered "best practice" to wrap quotes around font names that contain multiple words in the CSS font-family property, like this:

font-family: "Arial Narrow", Arial, Helvetica, sans-serif;

For the heck of it, I tried removing the quotes from "Arial Narrow" and Safari and Firefox don't have any problem rendering it.

So, is there any logic to this rule of thumb, or is it just a myth? Was it an issue with older browsers that no longer applies to the current versions? I've been doing this for so long that I never stopped to think if it was actually necessary.

The CSS 2.1 spec tells us that:

Font family names must either be given quoted as strings, or unquoted as a sequence of one or more identifiers. This means most punctuation characters and digits at the start of each token must be escaped in unquoted font family names.

It goes on to say:

If a sequence of identifiers is given as a font family name, the computed value is the name converted to a string by joining all the identifiers in the sequence by single spaces.

To avoid mistakes in escaping, it is recommended to quote font family names that contain white space, digits, or punctuation characters other than hyphens:

So yes, there is a difference, but one that's unlikely to cause any problems. Personally, I have always quoted font names when they contain spaces. In a few (presumably very rare) cases, the quotes are absolutely required:

Font family names that happen to be the same as a keyword value ('inherit', 'serif', 'sans-serif', 'monospace', 'fantasy', and 'cursive') must be quoted to prevent confusion with the keywords with the same names. The keywords 'initial' and 'default' are reserved for future use and must also be quoted when used as font names.

Also note that punctuation such as / or ! within an identifier may also need to be quoted or escaped.

According to the CSS Fonts Module Level 3 spec of October 2013, "font family names other than generic families must either be given quoted as strings, or unquoted as a sequence of one or more identifiers". So you DO NOT need to enclose them in quotes.

However, if you don't "most punctuation characters and digits at the start of each token must be escaped". To avoid escaping mistakes, the W3C actually recommends to quote font family names containing white space, digits, punctuation or keyword values (‘inherit’, ‘serif’, etc.).

The generic font family names (‘serif’, ‘sans-serif’, ‘cursive’, ‘fantasy’, and ‘monospace’) MUST NOT be quoted as they are actually keywords.