According to the W3C specifications:

A string cannot directly contain a newline. To include a newline in a string, use an escape representing the line feed character in ISO-10646 (U+000A), such as "\A" or "\00000a".

[...]

It is possible to break strings over several lines, for aesthetic or other reasons, but in such a case the newline itself has to be escaped with a backslash (\).

Noncompliant Code Example

a {
  content: "first
    second";
}

Compliant Solution

a {
  content: "first\Asecond";
}

See