Describe the bug

People write CSS like this for IE:

a::after {
    display: none \9;
}

Tools such as esbuild transform this into the following equivalent CSS (the \9 has been transformed into a \ followed by a tab character):

a::after {
    display: none \	;
}

However, cssnano incorrectly minifies this code into this CSS, which contains a syntax error:

a:after{display:none \}

This is a bug with cssnano, not with esbuild.

Expected behaviour

I expect cssnano to preserve escaped characters in CSS identifiers, and to not turn valid CSS code into code with a syntax error. A tab character in an identifier should either be printed as \ or as \9 (of course if the tab character is printed as \9, then an additional whitespace character may be needed to avoid the hex digit 9 accidentally merging with any following characters that happen to be hex digits). You can read more about how character escapes in CSS syntax work here: https://www.w3.org/TR/css-syntax-3/#consume-escaped-code-point

Steps to reproduce

1. Open https://cssnano.github.io/cssnano/playground/ in a browser

2. Paste the following CSS into the left textarea:

   a::after {
       display: none \	;
   }

3. Click the Minimize button

Version

https://cssnano.github.io/cssnano/playground/

Preset

(no preset)

Environment

https://cssnano.github.io/cssnano/playground/

Package details

https://cssnano.github.io/cssnano/playground/

Additional context

See also evanw/esbuild#3929.