diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56819570..b5162f63 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 1.27.0
+## 1.28.0
 
 ### Dart API
 
@@ -7,6 +7,85 @@
 
 [HWB]: https://en.wikipedia.org/wiki/HWB_color_model
 
+## 1.27.0
+
+* Adds an overload to `map.merge()` that supports merging a nested map.
+
+  `map.merge($map1, $keys..., $map2)`: The `$keys` form a path to the nested map
+  in `$map1`, into which `$map2` gets merged.
+
+  See [the Sass documentation][map-merge] for more details.
+
+  [map-merge]: https://sass-lang.com/documentation/modules/map#merge
+
+* Adds an overloaded `map.set()` function.
+
+  `map.set($map, $key, $value)`: Adds to or updates `$map` with the specified
+  `$key` and `$value`.
+
+  `map.set($map, $keys..., $value)`: Adds to or updates a map that is nested
+  within `$map`. The `$keys` form a path to the nested map in `$map`, into
+  which `$value` is inserted.
+
+  See [the Sass documentation][map-set] for more details.
+
+  [map-set]: https://sass-lang.com/documentation/modules/map#set
+
+* Add support for nested maps to `map.get()`.
+  For example, `map.get((a: (b: (c: d))), a, b, c)` would return `d`.
+  See [the documentation][map-get] for more details.
+
+  [map-get]: https://sass-lang.com/documentation/modules/map#get
+
+* Add support for nested maps in `map.has-key`.
+  For example, `map.has-key((a: (b: (c: d))), a, b, c)` would return true.
+  See [the documentation][map-has-key] for more details.
+
+  [map-has-key]: https://sass-lang.com/documentation/modules/map#has-key
+
+* Add a `map.deep-merge()` function. This works like `map.merge()`, except that
+  nested map values are *also* recursively merged. For example:
+
+  ```
+  map.deep-merge(
+    (color: (primary: red, secondary: blue),
+    (color: (secondary: teal)
+  ) // => (color: (primary: red, secondary: teal))
+  ```
+
+  See [the Sass documentation][map-deep-merge] for more details.
+
+  [map-deep-merge]: https://sass-lang.com/documentation/modules/map#deep-merge
+
+* Add a `map.deep-remove()` function. This allows you to remove keys from
+  nested maps by passing multiple keys. For example:
+
+  ```
+  map.deep-remove(
+    (color: (primary: red, secondary: blue)),
+    color, primary
+  ) // => (color: (secondary: blue))
+  ```
+
+  See [the Sass documentation][map-deep-remove] for more details.
+
+  [map-deep-remove]: https://sass-lang.com/documentation/modules/map#deep-remove
+
+* Fix a bug where custom property values in plain CSS were being parsed as
+  normal property values.
+
+### Dart API
+
+* Add a `Value.tryMap()` function which returns the `Value` as a `SassMap` if
+  it's a valid map, or `null` otherwise. This allows function authors to safely
+  retrieve maps even if they're internally stored as empty lists, without having
+  to catch exceptions from `Value.assertMap()`.
+
+## 1.26.12
+
+* Fix a bug where nesting properties beneath a Sass-syntax custom property
+  (written as `#{--foo}: ...`) would crash.
+
 ## 1.26.11
 
 * **Potentially breaking bug fix:** `selector.nest()` now throws an error
@@ -20,6 +99,9 @@
 
 * Don't crash when writing `Infinity` in JS mode.
 
+* Produce a better error message for positional arguments following named
+  arguments.
+
 ## 1.26.10
 
 * Fixes a bug where two adjacent combinators could cause an error.
diff --git a/bin/sass.dart b/bin/sass.dart
index f4391595..d74f39c6 100644
--- a/bin/sass.dart
+++ b/bin/sass.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:isolate';
 
 import 'package:path/path.dart' as p;
diff --git a/lib/sass.dart b/lib/sass.dart
index 992e1285..e45a610d 100644
--- a/lib/sass.dart
+++ b/lib/sass.dart
@@ -5,8 +5,6 @@
 /// We strongly recommend importing this library with the prefix `sass`.
 library sass;
 
-import 'dart:async';
-
 import 'package:source_maps/source_maps.dart';
 
 import 'src/async_import_cache.dart';
diff --git a/lib/src/ast/css/declaration.dart b/lib/src/ast/css/declaration.dart
index 01b26c3a..1bd7279f 100644
--- a/lib/src/ast/css/declaration.dart
+++ b/lib/src/ast/css/declaration.dart
@@ -24,5 +24,16 @@ abstract class CssDeclaration extends CssNode {
   /// the variable was used. Otherwise, this is identical to [value.span].
   FileSpan get valueSpanForMap;
 
-  T accept<T>(CssVisitor<T> visitor) => visitor.visitCssDeclaration(this);
+  /// Returns whether this is a CSS Custom Property declaration.
+  bool get isCustomProperty;
+
+  /// Whether this is was originally parsed as a custom property declaration, as
+  /// opposed to using something like `#{--foo}: ...` to cause it to be parsed
+  /// as a normal Sass declaration.
+  ///
+  /// If this is `true`, [isCustomProperty] will also be `true` and [value] will
+  /// contain a [SassString].
+  bool get parsedAsCustomProperty;
+
+  T accept<T>(CssVisitor<T> visitor);
 }
diff --git a/lib/src/ast/css/modifiable/declaration.dart b/lib/src/ast/css/modifiable/declaration.dart
index fb7dc2b3..e4bb7fc4 100644
--- a/lib/src/ast/css/modifiable/declaration.dart
+++ b/lib/src/ast/css/modifiable/declaration.dart
@@ -2,6 +2,7 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'package:meta/meta.dart';
 import 'package:source_span/source_span.dart';
 
 import '../../../value.dart';
@@ -15,13 +16,32 @@ class ModifiableCssDeclaration extends ModifiableCssNode
     implements CssDeclaration {
   final CssValue<String> name;
   final CssValue<Value> value;
+  final bool parsedAsCustomProperty;
   final FileSpan valueSpanForMap;
   final FileSpan span;
 
+  bool get isCustomProperty => name.value.startsWith('--');
+
+  /// Returns a new CSS declaration with the given properties.
   ModifiableCssDeclaration(this.name, this.value, this.span,
-      {FileSpan valueSpanForMap})
-      : valueSpanForMap = valueSpanForMap ?? span;
+      {@required bool parsedAsCustomProperty, FileSpan valueSpanForMap})
+      : parsedAsCustomProperty = parsedAsCustomProperty,
+        valueSpanForMap = valueSpanForMap ?? span {
+    if (parsedAsCustomProperty) {
+      if (!isCustomProperty) {
+        throw ArgumentError(
+            'parsedAsCustomProperty must be false if name doesn\'t begin with '
+            '"--".');
+      } else if (value.value is! SassString) {
+        throw ArgumentError(
+            'If parsedAsCustomProperty is true, value must contain a SassString '
+            '(was `$value` of type ${value.value.runtimeType}).');
+      }
+    }
+  }
 
   T accept<T>(ModifiableCssVisitor<T> visitor) =>
       visitor.visitCssDeclaration(this);
+
+  String toString() => "$name: $value;";
 }
diff --git a/lib/src/ast/sass/statement/declaration.dart b/lib/src/ast/sass/statement/declaration.dart
index 269d4080..6d6240ec 100644
--- a/lib/src/ast/sass/statement/declaration.dart
+++ b/lib/src/ast/sass/statement/declaration.dart
@@ -6,6 +6,7 @@ import 'package:source_span/source_span.dart';
 
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
+import '../expression/string.dart';
 import '../interpolation.dart';
 import '../statement.dart';
 import 'parent.dart';
@@ -20,10 +21,24 @@ class Declaration extends ParentStatement {
 
   final FileSpan span;
 
+  /// Returns whether this is a CSS Custom Property declaration.
+  ///
+  /// Note that this can return `false` for declarations that will ultimately be
+  /// serialized as custom properties if they aren't *parsed as* custom
+  /// properties, such as `#{--foo}: ...`.
+  ///
+  /// If this is `true`, then `value` will be a [StringExpression].
+  bool get isCustomProperty => name.initialPlain.startsWith('--');
+
   Declaration(this.name, this.span, {this.value, Iterable<Statement> children})
-      : super(children = children == null ? null : List.unmodifiable(children));
+      : super(
+            children = children == null ? null : List.unmodifiable(children)) {
+    if (isCustomProperty && value is! StringExpression) {
+      throw ArgumentError(
+          'Declarations whose names begin with "--" must have StringExpression '
+          'values (was `${value}` of type ${value.runtimeType}).');
+    }
+  }
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitDeclaration(this);
-
-  String toString() => "$name: $value;";
 }
diff --git a/lib/src/async_compile.dart b/lib/src/async_compile.dart
index 855f0f05..cc354a8c 100644
--- a/lib/src/async_compile.dart
+++ b/lib/src/async_compile.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:convert';
 
 import 'package:path/path.dart' as p;
diff --git a/lib/src/async_environment.dart b/lib/src/async_environment.dart
index 6a95da31..c8d73acc 100644
--- a/lib/src/async_environment.dart
+++ b/lib/src/async_environment.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:collection';
 
 import 'package:meta/meta.dart';
diff --git a/lib/src/async_import_cache.dart b/lib/src/async_import_cache.dart
index 03142a13..8725200a 100644
--- a/lib/src/async_import_cache.dart
+++ b/lib/src/async_import_cache.dart
@@ -2,8 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
-
 import 'package:collection/collection.dart';
 import 'package:path/path.dart' as p;
 import 'package:tuple/tuple.dart';
diff --git a/lib/src/compile.dart b/lib/src/compile.dart
index ea2793fd..0a0ecd5e 100644
--- a/lib/src/compile.dart
+++ b/lib/src/compile.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_compile.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 193c9bc1905022e881636f4d6359c906146abcb3
+// Checksum: bca3a79dd4a5c3905b07003b123172f3c876d2de
 //
 // ignore_for_file: unused_import
 
diff --git a/lib/src/environment.dart b/lib/src/environment.dart
index 2e1cf4e1..f9d6eaa9 100644
--- a/lib/src/environment.dart
+++ b/lib/src/environment.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: d304e1c5208019bca99df0678c3fdb4d09776a2b
+// Checksum: 9f4ee98a1c9e90d8d5277e0c2b0355460cda8788
 //
 // ignore_for_file: unused_import
 
diff --git a/lib/src/executable/compile_stylesheet.dart b/lib/src/executable/compile_stylesheet.dart
index 19a505e6..6e799118 100644
--- a/lib/src/executable/compile_stylesheet.dart
+++ b/lib/src/executable/compile_stylesheet.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:convert';
 
 import 'package:path/path.dart' as p;
diff --git a/lib/src/executable/repl.dart b/lib/src/executable/repl.dart
index 642d5984..cbf6097d 100644
--- a/lib/src/executable/repl.dart
+++ b/lib/src/executable/repl.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:math' as math;
 
 import 'package:cli_repl/cli_repl.dart';
diff --git a/lib/src/functions/map.dart b/lib/src/functions/map.dart
index 9e522411..76a1f228 100644
--- a/lib/src/functions/map.dart
+++ b/lib/src/functions/map.dart
@@ -7,6 +7,7 @@ import 'dart:collection';
 import 'package:collection/collection.dart';
 
 import '../callable.dart';
+import '../exception.dart';
 import '../module/built_in.dart';
 import '../value.dart';
 
@@ -21,19 +22,89 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass map module.
-final module = BuiltInModule("map",
-    functions: [_get, _merge, _remove, _keys, _values, _hasKey]);
+final module = BuiltInModule("map", functions: [
+  _get,
+  _set,
+  _merge,
+  _remove,
+  _keys,
+  _values,
+  _hasKey,
+  _deepMerge,
+  _deepRemove
+]);
 
-final _get = _function("get", r"$map, $key", (arguments) {
+final _get = _function("get", r"$map, $key, $keys...", (arguments) {
   var map = arguments[0].assertMap("map");
-  var key = arguments[1];
-  return map.contents[key] ?? sassNull;
+  var keys = [arguments[1], ...arguments[2].asList];
+  for (var key in keys.take(keys.length - 1)) {
+    var value = map.contents[key];
+    if (value is SassMap) {
+      map = value;
+    } else {
+      return sassNull;
+    }
+  }
+  return map.contents[keys.last] ?? sassNull;
 });
 
-final _merge = _function("merge", r"$map1, $map2", (arguments) {
+final _set = BuiltInCallable.overloadedFunction("set", {
+  r"$map, $key, $value": (arguments) {
+    var map = arguments[0].assertMap("map");
+    return _modify(map, [arguments[1]], (_) => arguments[2]);
+  },
+  r"$map, $args...": (arguments) {
+    var map = arguments[0].assertMap("map");
+    var args = arguments[1].asList;
+    if (args.isEmpty) {
+      throw SassScriptException("Expected \$args to contain a key.");
+    } else if (args.length == 1) {
+      throw SassScriptException("Expected \$args to contain a value.");
+    }
+    return _modify(map, args.sublist(0, args.length - 1), (_) => args.last);
+  },
+});
+
+final _merge = BuiltInCallable.overloadedFunction("merge", {
+  r"$map1, $map2": (arguments) {
+    var map1 = arguments[0].assertMap("map1");
+    var map2 = arguments[1].assertMap("map2");
+    return SassMap({...map1.contents, ...map2.contents});
+  },
+  r"$map1, $args...": (arguments) {
+    var map1 = arguments[0].assertMap("map1");
+    var args = arguments[1].asList;
+    if (args.isEmpty) {
+      throw SassScriptException("Expected \$args to contain a key.");
+    } else if (args.length == 1) {
+      throw SassScriptException("Expected \$args to contain a map.");
+    }
+    var map2 = args.last.assertMap("map2");
+    return _modify(map1, args.take(args.length - 1), (oldValue) {
+      var nestedMap = oldValue?.tryMap();
+      if (nestedMap == null) return map2;
+      return SassMap({...nestedMap.contents, ...map2.contents});
+    });
+  },
+});
+
+final _deepMerge = _function("deep-merge", r"$map1, $map2", (arguments) {
   var map1 = arguments[0].assertMap("map1");
   var map2 = arguments[1].assertMap("map2");
-  return SassMap({...map1.contents, ...map2.contents});
+  return _deepMergeImpl(map1, map2);
+});
+
+final _deepRemove =
+    _function("deep-remove", r"$map, $key, $keys...", (arguments) {
+  var map = arguments[0].assertMap("map");
+  var keys = [arguments[1], ...arguments[2].asList];
+  return _modify(map, keys.take(keys.length - 1), (value) {
+    var nestedMap = value?.tryMap();
+    if (nestedMap?.contents?.containsKey(keys.last) ?? false) {
+      return SassMap(Map.of(nestedMap.contents)..remove(keys.last));
+    }
+    return value;
+  });
 });
 
 final _remove = BuiltInCallable.overloadedFunction("remove", {
@@ -67,12 +138,100 @@ final _values = _function(
     (arguments) => SassList(
         arguments[0].assertMap("map").contents.values, ListSeparator.comma));
 
-final _hasKey = _function("has-key", r"$map, $key", (arguments) {
+final _hasKey = _function("has-key", r"$map, $key, $keys...", (arguments) {
   var map = arguments[0].assertMap("map");
-  var key = arguments[1];
-  return SassBoolean(map.contents.containsKey(key));
+  var keys = [arguments[1], ...arguments[2].asList];
+  for (var key in keys.take(keys.length - 1)) {
+    var value = map.contents[key];
+    if (value is SassMap) {
+      map = value;
+    } else {
+      return sassFalse;
+    }
+  }
+  return SassBoolean(map.contents.containsKey(keys.last));
 });
 
+/// Updates the specified value in [map] by applying the [modify] callback to
+/// it, then returns the resulting map.
+///
+/// If more than one key is provided, this means the map targeted for update is
+/// nested within [map]. The multiple [keys] form a path of nested maps that
+/// leads to the targeted map. If any value along the path is not a map, and
+/// `modify(null)` returns null, this inserts a new map at that key and
+/// overwrites the current value. Otherwise, this fails and returns [map] with
+/// no changes.
+///
+/// If no keys are provided, this passes [map] directly to modify and returns
+/// the result.
+Value _modify(SassMap map, Iterable<Value> keys, Value modify(Value old)) {
+  var keyIterator = keys.iterator;
+  SassMap _modifyNestedMap(SassMap map, [Value newValue]) {
+    var mutableMap = Map.of(map.contents);
+    var key = keyIterator.current;
+
+    if (!keyIterator.moveNext()) {
+      mutableMap[key] = newValue ?? modify(mutableMap[key]);
+      return SassMap(mutableMap);
+    }
+
+    var nestedMap = mutableMap[key]?.tryMap();
+    if (nestedMap == null) {
+      // We pass null to `modify` here to indicate there's no existing value.
+      newValue = modify(null);
+      if (newValue == null) return SassMap(mutableMap);
+    }
+
+    nestedMap ??= const SassMap.empty();
+    mutableMap[key] = _modifyNestedMap(nestedMap, newValue);
+    return SassMap(mutableMap);
+  }
+
+  return keyIterator.moveNext() ? _modifyNestedMap(map) : modify(map);
+}
+
+/// Merges [map1] and [map2], with values in [map2] taking precedence.
+///
+/// If both [map1] and [map2] have a map value associated with the same key,
+/// this recursively merges those maps as well.
+SassMap _deepMergeImpl(SassMap map1, SassMap map2) {
+  if (map2.contents.isEmpty) return map1;
+
+  // Avoid making a mutable copy of `map2` if it would totally overwrite `map1`
+  // anyway.
+  var mutable = false;
+  var result = map2.contents;
+  void _ensureMutable() {
+    if (mutable) return;
+    mutable = true;
+    result = Map.of(result);
+  }
+
+  // Because values in `map2` take precedence over `map1`, we just check if any
+  // entires in `map1` don't have corresponding keys in `map2`, or if they're
+  // maps that need to be merged in their own right.
+  map1.contents.forEach((key, value) {
+    var resultValue = result[key];
+    if (resultValue == null) {
+      _ensureMutable();
+      result[key] = value;
+    } else {
+      var resultMap = resultValue.tryMap();
+      var valueMap = value.tryMap();
+
+      if (resultMap != null && valueMap != null) {
+        var merged = _deepMergeImpl(valueMap, resultMap);
+        if (identical(merged, resultMap)) return;
+
+        _ensureMutable();
+        result[key] = merged;
+      }
+    }
+  });
+
+  return mutable ? SassMap(result) : map2;
+}
+
 /// Like [new BuiltInCallable.function], but always sets the URL to `sass:map`.
 BuiltInCallable _function(
         String name, String arguments, Value callback(List<Value> arguments)) =>
diff --git a/lib/src/import_cache.dart b/lib/src/import_cache.dart
index c86a1c9a..13887aca 100644
--- a/lib/src/import_cache.dart
+++ b/lib/src/import_cache.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_import_cache.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 8f54034c56e0d38fc8c90ad4d5f017628cab6190
+// Checksum: 5293a11e290c86829547ddd982ee3b1b1536dc73
 //
 // ignore_for_file: unused_import
 
diff --git a/lib/src/importer/node/interface.dart b/lib/src/importer/node/interface.dart
index c7228bde..4ae9afb8 100644
--- a/lib/src/importer/node/interface.dart
+++ b/lib/src/importer/node/interface.dart
@@ -2,8 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
-
 import 'package:tuple/tuple.dart';
 
 class NodeImporter {
diff --git a/lib/src/io/interface.dart b/lib/src/io/interface.dart
index 48815101..2835ee87 100644
--- a/lib/src/io/interface.dart
+++ b/lib/src/io/interface.dart
@@ -2,8 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
-
 import 'package:watcher/watcher.dart';
 
 /// An output sink that writes to this process's standard error.
diff --git a/lib/src/parse/stylesheet.dart b/lib/src/parse/stylesheet.dart
index da95981b..3fea27a0 100644
--- a/lib/src/parse/stylesheet.dart
+++ b/lib/src/parse/stylesheet.dart
@@ -503,8 +503,11 @@ abstract class StylesheetParser extends Parser {
   /// This is only used in contexts where declarations are allowed but style
   /// rules are not, such as nested declarations. Otherwise,
   /// [_declarationOrStyleRule] is used instead.
-  @protected
-  Statement _propertyOrVariableDeclaration() {
+  ///
+  /// If [parseCustomProperties] is `true`, properties that begin with `--` will
+  /// be parsed using custom property parsing rules.
+  Statement _propertyOrVariableDeclaration(
+      {bool parseCustomProperties = true}) {
     var start = scanner.state;
 
     Interpolation name;
@@ -533,6 +536,13 @@ abstract class StylesheetParser extends Parser {
 
     whitespace();
     scanner.expectChar($colon);
+
+    if (parseCustomProperties && name.initialPlain.startsWith('--')) {
+      var value = _interpolatedDeclarationValue();
+      expectStatementSeparator("custom property");
+      return Declaration(name, scanner.spanFrom(start), value: value);
+    }
+
     whitespace();
 
     if (lookingAtChildren()) {
@@ -562,7 +572,7 @@ abstract class StylesheetParser extends Parser {
   /// Consumes a statement that's allowed within a declaration.
   Statement _declarationChild() {
     if (scanner.peekChar() == $at) return _declarationAtRule();
-    return _propertyOrVariableDeclaration();
+    return _propertyOrVariableDeclaration(parseCustomProperties: false);
   }
 
   // ## At Rules
@@ -1581,7 +1591,8 @@ relase. For details, see http://bit.ly/moz-document.
           break;
         }
       } else if (named.isNotEmpty) {
-        scanner.expect("...");
+        error("Positional arguments must come before keyword arguments.",
+            expression.span);
       } else {
         positional.add(expression);
       }
diff --git a/lib/src/sync_package_resolver/node.dart b/lib/src/sync_package_resolver/node.dart
index 7affa204..c0984dce 100644
--- a/lib/src/sync_package_resolver/node.dart
+++ b/lib/src/sync_package_resolver/node.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 class SyncPackageResolver {
   static final _error =
       UnsupportedError('SyncPackageResolver is not supported in JS.');
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 4de346a6..69b96a50 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -2,7 +2,6 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:async';
 import 'dart:math' as math;
 
 import 'package:charcode/charcode.dart';
diff --git a/lib/src/value.dart b/lib/src/value.dart
index 320fe829..1bd532ca 100644
--- a/lib/src/value.dart
+++ b/lib/src/value.dart
@@ -98,6 +98,8 @@ abstract class Value implements ext.Value {
   SassMap assertMap([String name]) =>
       throw _exception("$this is not a map.", name);
 
+  SassMap tryMap() => null;
+
   SassNumber assertNumber([String name]) =>
       throw _exception("$this is not a number.", name);
 
diff --git a/lib/src/value/external/value.dart b/lib/src/value/external/value.dart
index 50a0ea7c..2089726c 100644
--- a/lib/src/value/external/value.dart
+++ b/lib/src/value/external/value.dart
@@ -104,6 +104,10 @@ abstract class Value {
   /// (without the `$`). It's used for error reporting.
   SassMap assertMap([String name]);
 
+  /// Returns [this] as a [SassMap] if it is one (including empty lists, which
+  /// count as empty maps) or returns `null` if it's not.
+  SassMap tryMap();
+
   /// Throws a [SassScriptException] if [this] isn't a number.
   ///
   /// If this came from a function argument, [name] is the argument name
diff --git a/lib/src/value/list.dart b/lib/src/value/list.dart
index fc09eceb..1960e0c5 100644
--- a/lib/src/value/list.dart
+++ b/lib/src/value/list.dart
@@ -46,6 +46,8 @@ class SassList extends Value implements ext.SassList {
   SassMap assertMap([String name]) =>
       asList.isEmpty ? const SassMap.empty() : super.assertMap(name);
 
+  SassMap tryMap() => asList.isEmpty ? const SassMap.empty() : null;
+
   bool operator ==(Object other) =>
       (other is SassList &&
           other.separator == separator &&
diff --git a/lib/src/value/map.dart b/lib/src/value/map.dart
index 9e187768..971b858c 100644
--- a/lib/src/value/map.dart
+++ b/lib/src/value/map.dart
@@ -32,6 +32,8 @@ class SassMap extends Value implements ext.SassMap {
 
   SassMap assertMap([String name]) => this;
 
+  SassMap tryMap() => this;
+
   bool operator ==(Object other) =>
       (other is SassMap && mapEquals(other.contents, contents)) ||
       (contents.isEmpty && other is SassList && other.asList.isEmpty);
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index 56a25b8f..4cb43ad1 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -1039,8 +1039,9 @@ class _EvaluateVisitor
     if (cssValue != null &&
         (!cssValue.value.isBlank || _isEmptyList(cssValue.value))) {
       _parent.addChild(ModifiableCssDeclaration(name, cssValue, node.span,
+          parsedAsCustomProperty: node.isCustomProperty,
           valueSpanForMap: _expressionNode(node.value)?.span));
-    } else if (name.value.startsWith('--')) {
+    } else if (name.value.startsWith('--') && node.children == null) {
       throw _exception(
           "Custom property values may not be empty.", node.value.span);
     }
@@ -2559,6 +2560,7 @@ class _EvaluateVisitor
 
   Future<void> visitCssDeclaration(CssDeclaration node) async {
     _parent.addChild(ModifiableCssDeclaration(node.name, node.value, node.span,
+        parsedAsCustomProperty: node.isCustomProperty,
         valueSpanForMap: node.valueSpanForMap));
   }
 
diff --git a/lib/src/visitor/clone_css.dart b/lib/src/visitor/clone_css.dart
index 95211f24..ae4fe115 100644
--- a/lib/src/visitor/clone_css.dart
+++ b/lib/src/visitor/clone_css.dart
@@ -44,6 +44,7 @@ class _CloneCssVisitor implements CssVisitor<ModifiableCssNode> {
 
   ModifiableCssDeclaration visitCssDeclaration(CssDeclaration node) =>
       ModifiableCssDeclaration(node.name, node.value, node.span,
+          parsedAsCustomProperty: node.parsedAsCustomProperty,
           valueSpanForMap: node.valueSpanForMap);
 
   ModifiableCssImport visitCssImport(CssImport node) =>
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index 8dd10751..d8e6e71f 100644
--- a/lib/src/visitor/evaluate.dart
+++ b/lib/src/visitor/evaluate.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: f6fe6645ccec58216ef623851bd2594de291a360
+// Checksum: 651a7e9f78b68bfd440241304301cf78711553a4
 //
 // ignore_for_file: unused_import
 
@@ -1041,8 +1041,9 @@ class _EvaluateVisitor
     if (cssValue != null &&
         (!cssValue.value.isBlank || _isEmptyList(cssValue.value))) {
       _parent.addChild(ModifiableCssDeclaration(name, cssValue, node.span,
+          parsedAsCustomProperty: node.isCustomProperty,
           valueSpanForMap: _expressionNode(node.value)?.span));
-    } else if (name.value.startsWith('--')) {
+    } else if (name.value.startsWith('--') && node.children == null) {
       throw _exception(
           "Custom property values may not be empty.", node.value.span);
     }
@@ -2541,6 +2542,7 @@ class _EvaluateVisitor
 
   void visitCssDeclaration(CssDeclaration node) {
     _parent.addChild(ModifiableCssDeclaration(node.name, node.value, node.span,
+        parsedAsCustomProperty: node.isCustomProperty,
         valueSpanForMap: node.valueSpanForMap));
   }
 
diff --git a/lib/src/visitor/serialize.dart b/lib/src/visitor/serialize.dart
index 450126f0..e5654fef 100644
--- a/lib/src/visitor/serialize.dart
+++ b/lib/src/visitor/serialize.dart
@@ -333,7 +333,10 @@ class _SerializeVisitor
     _write(node.name);
     _buffer.writeCharCode($colon);
 
-    if (_isParsedCustomProperty(node)) {
+    // If `node` is a custom property that was parsed as a normal Sass-syntax
+    // property (such as `#{--foo}: ...`), we serialize its value using the
+    // normal Sass property logic as well.
+    if (node.isCustomProperty && node.parsedAsCustomProperty) {
       _for(node.value, () {
         if (_isCompressed) {
           _writeFoldedValue(node);
@@ -355,20 +358,6 @@ class _SerializeVisitor
     }
   }
 
-  /// Returns whether [node] is a custom property that was parsed as a custom
-  /// property (rather than being dynamically generated, as in `#{--foo}: ...`).
-  ///
-  /// We only re-indent custom property values that were parsed as custom
-  /// properties, which we detect as unquoted strings. It's possible to have
-  /// false positives here, since someone could write `#{--foo}: unquoted`, but
-  /// that's unlikely enough that we can spare the extra time a no-op
-  /// reindenting will take.
-  bool _isParsedCustomProperty(CssDeclaration node) {
-    if (!node.name.value.startsWith("--")) return false;
-    var value = node.value.value;
-    return value is SassString && !value.hasQuotes;
-  }
-
   /// Emits the value of [node], with all newlines followed by whitespace
   void _writeFoldedValue(CssDeclaration node) {
     var scanner = StringScanner((node.value.value as SassString).text);
diff --git a/pubspec.yaml b/pubspec.yaml
index aa86787d..48777c61 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: sass
-version: 1.27.0-dev
+version: 1.28.0-dev
 description: A Sass implementation in Dart.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass
diff --git a/test/dart_api/value/boolean_test.dart b/test/dart_api/value/boolean_test.dart
index abdcea4b..87543148 100644
--- a/test/dart_api/value/boolean_test.dart
+++ b/test/dart_api/value/boolean_test.dart
@@ -31,6 +31,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
@@ -56,6 +57,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
diff --git a/test/dart_api/value/color_test.dart b/test/dart_api/value/color_test.dart
index d2eef137..4f88f60b 100644
--- a/test/dart_api/value/color_test.dart
+++ b/test/dart_api/value/color_test.dart
@@ -185,6 +185,7 @@ void main() {
       expect(value.assertBoolean, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
diff --git a/test/dart_api/value/function_test.dart b/test/dart_api/value/function_test.dart
index 5a0f5547..bca96635 100644
--- a/test/dart_api/value/function_test.dart
+++ b/test/dart_api/value/function_test.dart
@@ -31,6 +31,7 @@ void main() {
       expect(value.assertBoolean, throwsSassScriptException);
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
diff --git a/test/dart_api/value/list_test.dart b/test/dart_api/value/list_test.dart
index 92734498..9db1dce8 100644
--- a/test/dart_api/value/list_test.dart
+++ b/test/dart_api/value/list_test.dart
@@ -110,6 +110,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
@@ -140,6 +141,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
@@ -167,6 +169,7 @@ void main() {
 
     test("counts as an empty map", () {
       expect(value.assertMap().contents, isEmpty);
+      expect(value.tryMap().contents, isEmpty);
     });
 
     test("isn't any other type", () {
diff --git a/test/dart_api/value/map_test.dart b/test/dart_api/value/map_test.dart
index 2a8b19f3..23e241cb 100644
--- a/test/dart_api/value/map_test.dart
+++ b/test/dart_api/value/map_test.dart
@@ -128,6 +128,7 @@ void main() {
 
     test("is a map", () {
       expect(value.assertMap(), equals(value));
+      expect(value.tryMap(), equals(value));
     });
 
     test("isn't any other type", () {
diff --git a/test/dart_api/value/null_test.dart b/test/dart_api/value/null_test.dart
index b95a5377..6e3e9295 100644
--- a/test/dart_api/value/null_test.dart
+++ b/test/dart_api/value/null_test.dart
@@ -27,6 +27,7 @@ void main() {
     expect(value.assertColor, throwsSassScriptException);
     expect(value.assertFunction, throwsSassScriptException);
     expect(value.assertMap, throwsSassScriptException);
+    expect(value.tryMap(), isNull);
     expect(value.assertNumber, throwsSassScriptException);
     expect(value.assertString, throwsSassScriptException);
   });
diff --git a/test/dart_api/value/number_test.dart b/test/dart_api/value/number_test.dart
index 6e85ca46..f0640245 100644
--- a/test/dart_api/value/number_test.dart
+++ b/test/dart_api/value/number_test.dart
@@ -102,6 +102,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertString, throwsSassScriptException);
     });
   });
diff --git a/test/dart_api/value/string_test.dart b/test/dart_api/value/string_test.dart
index dfdae8b4..3e8c3642 100644
--- a/test/dart_api/value/string_test.dart
+++ b/test/dart_api/value/string_test.dart
@@ -37,6 +37,7 @@ void main() {
       expect(value.assertColor, throwsSassScriptException);
       expect(value.assertFunction, throwsSassScriptException);
       expect(value.assertMap, throwsSassScriptException);
+      expect(value.tryMap(), isNull);
       expect(value.assertNumber, throwsSassScriptException);
     });