string-no-newline
Disallow invalid newlines within strings.
a {
content: "first
second"; }/* ↑
* ↑
* The newline here */
The spec says this: "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'." And also: "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 (\)."