A URL is a pointer to a resource and is a functional notation
denoted by <url>. The Syntax of a <url> is:

<url> = url( <string> <url-modifier>* )

In addition to the Syntax defined above,a can sometimes be
written in other ways:

• For legacy reasons,a <url> can be written without quotation marks around the URL itself. This Syntax is specially-parsed,and
  produces a <url-token> rather than a function syntactically.
  [07001]

• Some CSS contexts,such as 07002,allow a <url> to be represented by a 07003 instead. This behaves identically to
  writing a url() function containing that string. Because these
  alternate ways of writing a <url> are not functional notations,they
  cannot accept any <url-modifier>s.

Note: The special parsing rules for the legacy quotation mark-less
<url> Syntax means that parentheses,whitespace characters,single
quotes (‘) and double quotes (“) appearing in a URL must be escaped
with a backslash,e.g. url(open\(parens),url(close\)parens).
Depending on the type of URL,it might also be possible to write these
characters as URL-escapes (e.g. url(open%28parens) or
url(close%29parens)) as described in[07004]. (If written as a
normal function containing a string,ordinary string escaping rules
apply; only newlines and the character used to quote the string need
to be escaped.)

在

07005

The url() function supports specifying additional <url-modifier>s,
which change the meaning or the interpretation of the URL somehow. A
<url-modifier> is either an 07006 or a 07007.

This specification does not define any <url-modifier>s,but other
specs may do so.

参见CSS Values and Units Module Level 3
Editor’s Draft,21 March 2016

>什么是< ident>的示例用法并在url()函数？
>< string>之间有什么区别？ ，< ident>，在url()中的功能？