Correct syntax rules for

files/en-us/web/css/ident/index.md

@@ -11,17 +11,30 @@ The **`<ident>`** [CSS](/en-US/docs/Web/CSS) [data type](/en-US/docs/Web/CSS/CSS
 
 ## Syntax
 
-The syntax of `<custom-ident>` is similar to CSS identifiers (such as property names), except that it is [case-sensitive](https://en.wikipedia.org/wiki/Case_sensitivity). It consists of one or more characters, where characters can be any of the following:
+A CSS identifier consists of one or more characters, where characters can be any of the following:
 
-- any alphabetical character (`A` to `Z`, or `a` to `z`),
-- any decimal digit (`0` to `9`),
-- a hyphen (`-`),
-- an underscore (`_`),
-- an escaped character (preceded by a backslash, `\`),
-- a [Unicode](https://en.wikipedia.org/wiki/Unicode) character (in the format of a backslash, `\`, followed by one to six hexadecimal digits, representing its Unicode code point)
+- any ASCII character `A-Z` and `a-z`
+- any decimal digit (`0` to `9`)
+- a hyphen (`-`)
+- an underscore (`_`)
+- any other Unicode character `U+00A0` and higher (that is, any other non-ASCII Unicode character)
+- an [escaped character](escaping_characters).
+
+Additionally, an identifier must not start with an unescaped digit, and must not start with an unescaped hyphen followed by an unescaped digit.
 
 Note that `id1`, `Id1`, `iD1` and `ID1` are all different identifiers as they are [case-sensitive](https://en.wikipedia.org/wiki/Case_sensitivity). On the other hand, as there are several ways to escape a character, `toto\?` and `toto\3F` are the same identifiers.
 
+### Escaping characters
+
+Any character except a hexadecimal digit (`[0-9a-fA-F]`) can be escaped by placing a backslash in front of it. For example, `&` can be escaped as `\&`.
+
+Any character can be escaped with a backslash followed by followed by one to six hexadecimal digits, representing the character's Unicode {{glossary("code point")}}. For example, `&` can be escaped as `\26`. In this situation, if the character following the escaped character is a hexadecimal digit, then either:
+
+- a space or other whitespace character must be placed after the Unicode code point, or:
+- the Unicode code point must be given in full, with all six digits.
+
+For example, the string `&123` could be escaped as `\26 123` or `\000026123`.
+
 ## Examples
 
 ### Valid identifiers
@@ -32,16 +45,18 @@ ground-level  A mix of alphanumeric characters and a dash
 -test         A dash followed by alphanumeric characters
 --toto        A custom-property like identifier
 _internal     An underscore followed by alphanumeric characters
-\22 toto      A Unicode character followed by a sequence of alphanumeric characters
+\22 toto      An escaped character followed by a sequence of alphanumeric characters
+\000022toto   The same as the previous example
 bili\.bob     A correctly escaped period
+🔥123         A non-ASCII character followed by numbers
 ```
 
 ### Invalid identifiers
 
 ```plain example-bad
 34rem     It must not start with a decimal digit.
 -12rad    It must not start with a dash followed by a decimal digit.
-bili.bob  Only alphanumeric characters, _, and - needn't be escaped.
+bili.bob  ASCII characters apart from alphanumerics must be escaped.
 'bilibob' This would be a {{CSSxRef("&lt;string&gt;")}}.
 "bilibob" This would be a {{CSSxRef("&lt;string&gt;")}}.
 ```