diff --git a/spec/formatting.md b/spec/formatting.md
index 85ea0288d1..c8179ef12d 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -197,17 +197,28 @@ the following steps are taken:
 
 1. If the _expression_ includes an _operand_, resolve its value.
    If this fails, use a _fallback value_ for the _expression_.
-2. Based on the _function_ starting sigil and _name_,
-   find the appropriate function implementation from the _function registry_.
-   If the registry does not define an implementation for this _name_,
+2. Resolve the _identifier_ of the _function_ and, based on the starting sigil,
+   find the appropriate function implementation to call.
+   If the implementation cannot find the function,
+   or if the _identifier_ includes a _namespace_ that the implementation does not support,
    emit an Unknown Function error
    and use a _fallback value_ for the _expression_.
-3. Resolve the _option_ values to a mapping of string identifiers to values.
+
+   Implementations are not required to implement _namespaces_ or installable
+   _function registries_.
+
+3. Resolve the _options_ to a mapping of string identifiers to values.
+   If _options_ is missing, the mapping will be empty.
    For each _option_:
-   - If its right-hand side successfully resolves to a value,
-     bind the _name_ of the _option_ to the resolved value in the mapping.
-   - Otherwise, do not bind the _name_ of the _option_ to any value in the mapping.
-4. Call the function implementation with the following arguments:
+   - Resolve the _identifier_ of the _option_.
+   - If the _option_'s _identifier_ already exists in the resolved mapping of _options_,
+     emit a Duplicate Option Name error.
+   - If the _option_'s right-hand side successfully resolves to a value,
+     bind the _identifier_ of the _option_ to the resolved value in the mapping.
+   - Otherwise, bind the _identifier_ of the _option_ to an unresolved value in the mapping.
+     (Note that an Unresolved Variable error will have been emitted.)
+4. Remove from the resolved mapping of _options_ any binding for which the value is an unresolved value.
+5. Call the function implementation with the following arguments:
 
    - The current _locale_.
    - The resolved mapping of _options_.
@@ -219,15 +230,20 @@ the following steps are taken:
    as long as reasonable precautions are taken to keep the function interface
    simple and minimal, and avoid introducing potential security vulnerabilities.
 
-   As implementations MAY allow custom functions to be defined by users,
-   their access to the _formatting context_ SHOULD be minimal and read-only,
-   and their execution time SHOULD be limited.
+   An implementation MAY define its own functions.
+   An implementation MAY allow custom functions to be defined by users.
+
+   Function access to the _formatting context_ MUST be minimal and read-only,
+   and execution time SHOULD be limited.
+   
+   Implementation-defined _functions_ SHOULD use an implementation-defined _namespace_.
 
 5. If the call succeeds,
    resolve the value of the _expression_ as the result of that function call.
    If the call fails or does not return a valid value,
    emit a Resolution error and use a _fallback value_ for the _expression_.
 
+
 ### Fallback Resolution
 
 A **_fallback value_** is the resolved value emitted when an _expression_ cannot be resolved.
@@ -255,13 +271,13 @@ The _fallback value_ depends on the contents of the _expression_:
   > Example: `$user`
 
 - _expression_ with no _operand_:
-  the _function_ starting sigil followed by its _name_
+  the _function_ starting sigil followed by its _identifier_
 
   > Examples: `:platform`, `+tag`, `-tag`
 
 - Otherwise: The U+FFFD REPLACEMENT CHARACTER `�` character
 
-_Option_ names and values are not included in the _fallback value_.
+_Option_ identifiers and values are not included in the _fallback value_.
 
 When an error occurs in an _expression_ with a _variable_ _operand_
 and the _variable_ refers to a local _declaration_,
@@ -817,7 +833,7 @@ These are divided into the following categories:
     > }}
     > ```
 
-  - A **Duplicate Option Name error** occurs when the same _name_
+  - A **Duplicate Option Name error** occurs when the same _identifier_
     appears on the left-hand side
     of more than one _option_ in the same _expression_.
 
diff --git a/spec/message.abnf b/spec/message.abnf
index bd340fe9f2..f4b610fe5c 100644
--- a/spec/message.abnf
+++ b/spec/message.abnf
@@ -23,8 +23,8 @@ annotation = (function *(s option)) / reserved / private-use
 
 literal = quoted / unquoted
 variable = "$" name
-function = (":" / "+" / "-") name
-option = name [s] "=" [s] (literal / variable)
+function = (":" / "+" / "-") identifier
+option = identifier [s] "=" [s] (literal / variable)
 
 ; reserved keywords are always lowercase
 input = %s"input"
@@ -45,13 +45,10 @@ quoted-char = %x0-5B         ; omit \
             / %x7D-D7FF      ; omit surrogates
             / %xE000-10FFFF
 
-; based on https://www.w3.org/TR/xml/#NT-Nmtoken,
-; but cannot start with U+002D HYPHEN-MINUS or U+003A COLON ":"
-unquoted = unquoted-start *name-char
+unquoted       = unquoted-start *(name-char / ":")
 unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
 
-
 ; reserve sigils for private-use by implementations
 private-use    = private-start reserved-body
 private-start  = "^" / "&"
@@ -70,15 +67,17 @@ reserved-char  = %x00-08        ; omit HTAB and LF
                / %x7E-D7FF      ; omit surrogates
                / %xE000-10FFFF
 
-; based on https://www.w3.org/TR/xml/#NT-Name,
-; but cannot start with U+003A COLON ":"
-name = name-start *name-char
+; identifier matches https://www.w3.org/TR/REC-xml-names/#NT-QName
+; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName
+identifier = [namespace ":"] name
+namespace  = name
+name       = name-start *name-char
 name-start = ALPHA / "_"
            / %xC0-D6 / %xD8-F6 / %xF8-2FF
            / %x370-37D / %x37F-1FFF / %x200C-200D
            / %x2070-218F / %x2C00-2FEF / %x3001-D7FF
            / %xF900-FDCF / %xFDF0-FFFD / %x10000-EFFFF
-name-char  = name-start / DIGIT / "-" / "." / ":"
+name-char  = name-start / DIGIT / "-" / "."
            / %xB7 / %x300-36F / %x203F-2040
 
 text-escape     = backslash ( backslash / "{" / "}" )
diff --git a/spec/syntax.md b/spec/syntax.md
index 1b68da8e60..9d2a3f9b94 100644
--- a/spec/syntax.md
+++ b/spec/syntax.md
@@ -169,7 +169,7 @@ external input value does not appear in a _declaration_.
 > [!Note]
 > These restrictions only apply to _declarations_.
 > A _placeholder_ or _selector_ can apply a different annotation to a _variable_
-> than one applied to the same _variable_ name in a _declaration_.
+> than one applied to the same _variable_ named in a _declaration_.
 > For example, this message is _valid_:
 > ```
 > {{
@@ -482,7 +482,7 @@ and vice versa.
 > {+button}Submit{-button} or {+link}cancel{-link}.
 > ```
 
-A _function_ consists of a prefix sigil followed by a _name_.
+A _function_ consists of a prefix sigil followed by an _identifier_.
 The following sigils are used for _functions_:
 
 - `:` for a _standalone_ function
@@ -497,8 +497,8 @@ _Options_ are not required.
 An **_<dfn>option</dfn>_** is a key-value pair
 containing a named argument that is passed to a _function_.
 
-An _option_ has a _name_ and a _value_.
-The _name_ is separated from the _value_ by an U+003D EQUALS SIGN `=` along with
+An _option_ has an _identifier_ and a _value_.
+The _identifier_ is separated from the _value_ by an U+003D EQUALS SIGN `=` along with
 optional whitespace.
 The value of an _option_ can be either a _literal_ or a _variable_.
 
@@ -506,7 +506,7 @@ Multiple _options_ are permitted in an _annotation_.
 Each _option_ is separated by whitespace.
 
 ```abnf
-option = name [s] "=" [s] (literal / variable)
+option = identifier [s] "=" [s] (literal / variable)
 ```
 
 > Examples of _functions_ with _options_
@@ -672,21 +672,65 @@ unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
 ```
 
-### Names
+### Names and Identifiers
 
-A **_<dfn>name</dfn>_** is an identifier for a _variable_ (prefixed with `$`),
+An **_<dfn>identifier</dfn>_** is a character sequence that
+identifies a _function_ or _option_.
+Each _identifier_ consists of a _name_ optionally preceeded by
+a _namespace_. 
+When present, the _namespace_ is separated from the _name_ by a
+U+003A COLON `:`.
+Built-in _functions_ and their _options_ do not have a _namespace_ identifier.
+
+_Function_ _identifiers_ are prefixed with `:`, `+`, or `-`.
+_Option_ _identifiers_ have no prefix.
+
+A **_<dfn>name</dfn>_** is a character sequence used in an _identifier_ 
+or as the name for for a _variable_.
+
+_Variable_ names are prefixed with `$`.
 for a _function_ (prefixed with `:`, `+` or `-`),
-or for an _option_ (these have no prefix).
-The namespace for _names_ is based on XML's [Name](https://www.w3.org/TR/xml/#NT-Name),
-with the restriction that it MUST NOT start with `:`,
-as that would conflict with the _function_ start character.
-Otherwise, the set of characters allowed in names is large.
+
+Valid content for _names_ is based on <cite>Namespaces in XML 1.0</cite>'s 
+[NCName](https://www.w3.org/TR/xml-names/#NT-NCName).
+This is different from XML's [Name](https://www.w3.org/TR/xml/#NT-Name)
+in that it MUST NOT contain a U+003A COLON `:`.
+Otherwise, the set of characters allowed in a _name_ is large.
+
+> [!NOTE]
+> _External variables_ can be passed in that are not valid _names_.
+> Such variables cannot be referenced in a _message_,
+> but are not otherwise errors.
+
+Examples:
+> A variable:
+>```
+>This has a {$variable}
+>```
+>A function:
+> ```
+> This has a {:function}
+> ```
+> An add-on function from the `icu` namespace:
+> ```
+> This has a {:icu:function}
+> ```
+> An option and an add-on option:
+> ```
+> This has {:options option=value icu:option=add_on}
+> ```
+
+Support for _namespaces_ and their interpretation is implementation-defined
+in this release.
 
 ```abnf
 variable = "$" name
-function = (":" / "+" / "-") name
+function = (":" / "+" / "-") identifier
+option = identifier [s] "=" [s] (literal / variable)
 
-name = name-start *name-char
+identifier = [namespace ":"] name
+namespace  = name
+name       = name-start *name-char
 name-start = ALPHA / "_"
            / %xC0-D6 / %xD8-F6 / %xF8-2FF
            / %x370-37D / %x37F-1FFF / %x200C-200D
@@ -696,11 +740,6 @@ name-char  = name-start / DIGIT / "-" / "." / ":"
            / %xB7 / %x300-36F / %x203F-2040
 ```
 
-> [!NOTE]
-> _External variables_ can be passed in that are not valid _names_.
-> Such variables cannot be referenced in a _message_,
-> but are not otherwise errors.
-
 ### Escape Sequences
 
 An **_<dfn>escape sequence</dfn>_** is a two-character sequence starting with