diff --git a/js-api-spec.ts b/js-api-spec.ts
index 3eb36453e6..4a48ef75dc 100644
--- a/js-api-spec.ts
+++ b/js-api-spec.ts
@@ -82,6 +82,12 @@ if (!fs.existsSync(specIndex)) {
 // `node_modules` directory.
 fs.copySync(p.resolve(specPath), p.join(sassPackagePath, 'js-api'));
 
+// Copy deprecations YAML so we can test against it.
+fs.copySync(
+  p.resolve(p.join(argv.sassSassRepo, 'spec/deprecations.yaml')),
+  p.join(sassPackagePath, 'deprecations.yaml')
+);
+
 fs.writeFileSync(
   p.join(sassPackagePath, 'package.json'),
   JSON.stringify({
diff --git a/js-api-spec/deprecations.node.test.ts b/js-api-spec/deprecations.node.test.ts
new file mode 100644
index 0000000000..b0e649f49a
--- /dev/null
+++ b/js-api-spec/deprecations.node.test.ts
@@ -0,0 +1,68 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import fs from 'fs';
+import yaml from 'js-yaml';
+import {deprecations, Deprecation, Version} from 'sass';
+
+describe('deprecation type', () => {
+  const deprecationsMap = deprecations as unknown as {
+    [key: string]: Deprecation;
+  };
+  const obsoleteDeprecations: {[key: string]: [string, string]} = {};
+  const activeDeprecations: {[key: string]: string} = {};
+  const futureDeprecations: Set<string> = new Set();
+  const data = yaml.load(
+    fs.readFileSync('js-api-spec/node_modules/sass/deprecations.yaml', 'utf8')
+  ) as {
+    [key: string]: {
+      'dart-sass':
+        | {status: 'future'}
+        | {status: 'active'; deprecated: string}
+        | {status: 'obsolete'; deprecated: string; obsolete: string};
+    };
+  };
+  for (const [id, deprecation] of Object.entries(data)) {
+    const dartSass = deprecation['dart-sass'];
+    if (dartSass.status === 'obsolete') {
+      obsoleteDeprecations[id] = [dartSass.deprecated, dartSass.obsolete];
+    } else if (dartSass.status === 'active') {
+      activeDeprecations[id] = dartSass.deprecated;
+    } else if (dartSass.status === 'future') {
+      futureDeprecations.add(id);
+    }
+  }
+
+  // These tests assume that the JS API being tested is backed by Dart Sass.
+  // If there's a JS API implementation in the future with a different compiler,
+  // then these tests shouldn't be run.
+  for (const [id, versions] of Object.entries(obsoleteDeprecations)) {
+    if (!versions) continue;
+    const [deprecatedIn, obsoleteIn] = versions;
+    it(`${id} deprecated in ${deprecatedIn} and obsolete in ${obsoleteIn}`, () => {
+      const deprecation = deprecationsMap[id];
+      expect(deprecation?.id).toBe(id);
+      expect(deprecation?.status).toBe('obsolete');
+      expect(deprecation?.deprecatedIn).toEqual(Version.parse(deprecatedIn));
+      expect(deprecation?.obsoleteIn).toEqual(Version.parse(obsoleteIn));
+    });
+  }
+
+  for (const [id, version] of Object.entries(activeDeprecations)) {
+    it(`${id} deprecated in ${version}`, () => {
+      const deprecation = deprecationsMap[id];
+      expect(deprecation?.id).toBe(id);
+      expect(deprecation?.status).toBe('active');
+      expect(deprecation?.deprecatedIn).toEqual(Version.parse(version));
+    });
+  }
+
+  for (const id of futureDeprecations) {
+    it(`${id} is a future deprecation`, () => {
+      const deprecation = deprecationsMap[id];
+      expect(deprecation?.id).toBe(id);
+      expect(deprecation?.status).toBe('future');
+    });
+  }
+});
diff --git a/js-api-spec/deprecations.test.ts b/js-api-spec/deprecations.test.ts
index fd343122a8..852c068f88 100644
--- a/js-api-spec/deprecations.test.ts
+++ b/js-api-spec/deprecations.test.ts
@@ -6,116 +6,12 @@ import {
   compileString,
   deprecations,
   Deprecation,
-  Deprecations,
   Importer,
   Version,
 } from 'sass';
 
 import {captureStdio, URL} from './utils';
 
-/**
- * Map from obsolete deprecation IDs to version pairs.
- *
- * The first version is the version this deprecation type was deprecated in,
- * while the second version is the version it was made obsolete in.
- */
-const obsoleteDeprecations: {[key in keyof Deprecations]?: [string, string]} =
-  {};
-
-/** Map from active deprecation IDs to the version they were deprecated in. */
-const activeDeprecations: {[key in keyof Deprecations]?: string} = {
-  'call-string': '0.0.0',
-  elseif: '1.3.2',
-  'moz-document': '1.7.2',
-  'relative-canonical': '1.14.2',
-  'new-global': '1.17.2',
-  'color-module-compat': '1.23.0',
-  'slash-div': '1.33.0',
-  'bogus-combinators': '1.54.0',
-  'strict-unary': '1.55.0',
-  'function-units': '1.56.0',
-  'duplicate-var-flags': '1.62.0',
-  'null-alpha': '1.62.3',
-  'abs-percent': '1.65.0',
-  'fs-importer-cwd': '1.73.0',
-  'color-4-api': '1.76.0',
-  'color-functions': '1.76.0',
-};
-
-/**
- * List of future deprecation IDs.
- *
- * This is only structured as an object to allow us to use a mapped object type
- * to ensure that all deprecation IDs listed here are included in the JS API
- * spec.
- */
-const futureDeprecations: {[key in keyof Deprecations]?: true} = {import: true};
-
-/**
- * This is a temporary synchronization check to ensure that any new deprecation
- * types are added to all five of these locations:
- * - lib/src/deprecation.dart in sass/dart-sass
- * - js-api-doc/deprecations.d.ts in sass/sass
- * - spec/js-api/deprecations.d.ts.md in sass/sass
- * - lib/src/deprecations.ts in sass/embedded-host-node
- * - js-api-spec/deprecations.test.ts in sass/sass-spec (this file)
- *
- * Work to replace these manual changes with generated code from a single
- * source-of-truth is tracked in sass/sass#3827
- */
-it('there are no extra or missing deprecation types', () => {
-  const expectedDeprecations = [
-    ...Object.keys(obsoleteDeprecations),
-    ...Object.keys(activeDeprecations),
-    ...Object.keys(futureDeprecations),
-    'user-authored',
-  ];
-  const actualDeprecations = Object.keys(deprecations);
-  const extraDeprecations = actualDeprecations.filter(
-    deprecation => !expectedDeprecations.includes(deprecation)
-  );
-  expect(extraDeprecations).toBeEmptyArray();
-  const missingDeprecations = expectedDeprecations.filter(
-    deprecation => !actualDeprecations.includes(deprecation)
-  );
-  expect(missingDeprecations).toBeEmptyArray();
-});
-
-describe('deprecation type', () => {
-  const deprecationsMap = deprecations as unknown as {
-    [key: string]: Deprecation;
-  };
-
-  for (const [id, versions] of Object.entries(obsoleteDeprecations)) {
-    if (!versions) continue;
-    const [deprecatedIn, obsoleteIn] = versions;
-    it(`${id} deprecated in ${deprecatedIn} and obsolete in ${obsoleteIn}`, () => {
-      const deprecation = deprecationsMap[id];
-      expect(deprecation?.id).toBe(id);
-      expect(deprecation?.status).toBe('obsolete');
-      expect(deprecation?.deprecatedIn).toEqual(Version.parse(deprecatedIn));
-      expect(deprecation?.obsoleteIn).toEqual(Version.parse(obsoleteIn));
-    });
-  }
-
-  for (const [id, version] of Object.entries(activeDeprecations)) {
-    it(`${id} deprecated in ${version}`, () => {
-      const deprecation = deprecationsMap[id];
-      expect(deprecation?.id).toBe(id);
-      expect(deprecation?.status).toBe('active');
-      expect(deprecation?.deprecatedIn).toEqual(Version.parse(version));
-    });
-  }
-
-  for (const [id] of Object.entries(futureDeprecations)) {
-    it(`${id} is a future deprecation`, () => {
-      const deprecation = deprecationsMap[id];
-      expect(deprecation?.id).toBe(id);
-      expect(deprecation?.status).toBe('future');
-    });
-  }
-});
-
 describe('a warning', () => {
   it('is emitted with no flags', done => {
     compileString('a { $b: c !global; }', {
diff --git a/js-api-spec/importer.node.test.ts b/js-api-spec/importer.node.test.ts
index a240ccc27c..e03ad18fb9 100644
--- a/js-api-spec/importer.node.test.ts
+++ b/js-api-spec/importer.node.test.ts
@@ -12,6 +12,7 @@ import {
 } from 'sass';
 
 import {sandbox} from './sandbox';
+import {spy} from './utils';
 
 it('avoids importer when canonicalize() returns null', () =>
   sandbox(dir => {
@@ -366,6 +367,30 @@ describe('FileImporter', () => {
       ).toThrowSassException({line: 0});
     });
   });
+
+  // Regression test for sass/dart-sass#2208.
+  it('imports the same relative url from different base urls as different files', () =>
+    sandbox(dir => {
+      const findFileUrl = spy((url, context) => {
+        return url === 'y' ? new URL('x.scss', context.containingUrl) : null;
+      });
+
+      dir.write({
+        'main.scss': '@import "sub1/test"; @import "sub1/sub2/test"',
+        'sub1/test.scss': '@import "y"',
+        'sub1/x.scss': 'x { from: sub1; }',
+        'sub1/sub2/test.scss': '@import "y"',
+        'sub1/sub2/x.scss': 'x { from: sub2; }',
+      });
+
+      expect(
+        compile(dir('main.scss'), {
+          importers: [{findFileUrl}],
+        }).css.toString()
+      ).toEqualIgnoringWhitespace('x { from: sub1; } x { from: sub2; }');
+
+      expect(findFileUrl).toHaveBeenCalledTimes(2);
+    }));
 });
 
 it(
diff --git a/js-api-spec/legacy/importer.node.test.ts b/js-api-spec/legacy/importer.node.test.ts
index 081d36f536..43f567fc24 100644
--- a/js-api-spec/legacy/importer.node.test.ts
+++ b/js-api-spec/legacy/importer.node.test.ts
@@ -178,6 +178,36 @@ describe('with contents', () => {
         }),
       }).stats.includedFiles
     ).toContain(p.resolve('bar')));
+
+  // Regression test for sass/dart-sass#2208.
+  it('imports the same relative url from different base urls as different files', () =>
+    sandbox(dir => {
+      const importer = spy((url: string, prev: string) => {
+        return url === 'x'
+          ? {
+              contents: `x {from: ${p.basename(p.dirname(prev))}}`,
+              file: p.resolve(p.dirname(prev), 'x.scss'),
+            }
+          : null;
+      });
+
+      dir.write({
+        'main.scss': '@import "sub1/test"; @import "sub1/sub2/test"',
+        'sub1/test.scss': '@import "x"',
+        'sub1/sub2/test.scss': '@import "x"',
+      });
+
+      expect(
+        sass
+          .renderSync({
+            file: dir('main.scss'),
+            importer,
+          })
+          .css.toString()
+      ).toEqualIgnoringWhitespace('x { from: sub1; } x { from: sub2; }');
+
+      expect(importer).toHaveBeenCalledTimes(2);
+    }));
 });
 
 describe('with a file redirect', () => {
diff --git a/spec/css/keyframes.hrx b/spec/css/keyframes.hrx
index 1e7648e4dc..3169f8f140 100644
--- a/spec/css/keyframes.hrx
+++ b/spec/css/keyframes.hrx
@@ -248,3 +248,48 @@ $a: b;
     c: d;
   }
 }
+
+<===>
+================================================================================
+<===> in_keyframe_block/unknown_at_rule/input.scss
+@keyframes a {
+  to {@b}
+}
+
+<===> in_keyframe_block/unknown_at_rule/output.css
+@keyframes a {
+  to {
+    @b;
+  }
+}
+
+<===>
+================================================================================
+<===> in_keyframe_block/known_at_rule/input.scss
+@keyframes a {
+  to {@media screen {b: c}}
+}
+
+<===> in_keyframe_block/known_at_rule/output.css
+@keyframes a {
+  to {
+    @media screen {
+      b: c;
+    }
+  }
+}
+
+<===>
+================================================================================
+<===> error/in_keyframe_block/style_rule/input.scss
+@keyframes a {
+  to {to {c: d}}
+}
+
+<===> error/in_keyframe_block/style_rule/error
+Error: Style rules may not be used within keyframe blocks.
+  ,
+2 |   to {to {c: d}}
+  |       ^^^^^^^^^
+  '
+  input.scss 2:7  root stylesheet
diff --git a/spec/directives/function.hrx b/spec/directives/function.hrx
new file mode 100644
index 0000000000..7c8c993088
--- /dev/null
+++ b/spec/directives/function.hrx
@@ -0,0 +1,94 @@
+<===> escaped/input.scss
+// Function names can be defined and referred to using escapes, which are
+// normalized.
+@function f\6Fo-bar() {@return 1}
+
+a {b: foo-b\61r()}
+
+<===> escaped/output.css
+a {
+  b: 1;
+}
+
+<===>
+================================================================================
+<===> custom_ident_name/input.scss
+@function --a() {@return 1}
+b {c: --a()}
+
+<===> custom_ident_name/output.css
+b {
+  c: 1;
+}
+
+<===> custom_ident_name/warning
+DEPRECATION WARNING on line 1, column 11 of input.scss: 
+Sass @function names beginning with -- are deprecated for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+  ,
+1 | @function --a() {@return 1}
+  |           ^^^
+  '
+
+<===>
+================================================================================
+<===> double_underscore_name/input.scss
+@function __a() {@return 1}
+b {c: __a()}
+
+<===> double_underscore_name/output.css
+b {
+  c: 1;
+}
+
+<===>
+================================================================================
+<===> custom_ident_call/input.scss
+@function __a() {@return 1}
+b {c: --a()}
+
+<===> custom_ident_call/output.css
+b {
+  c: 1;
+}
+
+<===> custom_ident_call/warning
+DEPRECATION WARNING: Sass @function names beginning with -- are deprecated for forward-compatibility with plain CSS functions.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+
+  ,
+2 | b {c: --a()}
+  |       ^^^
+  '
+    input.scss 2:7  root stylesheet
+
+<===>
+================================================================================
+<===> vendor_like_underscore/README.md
+Function names like `-moz-calc()` aren't allowed, but they are with underscores.
+
+<===>
+================================================================================
+<===> vendor_like_underscore/start/input.scss
+@function _moz-calc() {@return 1}
+b {c: _moz-calc()}
+
+<===> vendor_like_underscore/start/output.css
+b {
+  c: 1;
+}
+
+<===>
+================================================================================
+<===> vendor_like_underscore/middle/input.scss
+@function -moz_calc() {@return 1}
+b {c: -moz_calc()}
+
+<===> vendor_like_underscore/middle/output.css
+b {
+  c: 1;
+}
+
+
diff --git a/spec/directives/function/escaped.hrx b/spec/directives/function/escaped.hrx
deleted file mode 100644
index a43f229f71..0000000000
--- a/spec/directives/function/escaped.hrx
+++ /dev/null
@@ -1,11 +0,0 @@
-<===> input.scss
-// Function names can be defined and referred to using escapes, which are
-// normalized.
-@function f\6Fo-bar() {@return 1}
-
-a {b: foo-b\61r()}
-
-<===> output.css
-a {
-  b: 1;
-}
diff --git a/spec/directives/mixin.hrx b/spec/directives/mixin.hrx
new file mode 100644
index 0000000000..93e3568647
--- /dev/null
+++ b/spec/directives/mixin.hrx
@@ -0,0 +1,51 @@
+<===> custom_ident_name/input.scss
+@mixin --a {b: c}
+d {@include --a}
+
+<===> custom_ident_name/output.css
+d {
+  b: c;
+}
+
+<===> custom_ident_name/warning
+DEPRECATION WARNING on line 1, column 8 of input.scss: 
+Sass @mixin names beginning with -- are deprecated for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+  ,
+1 | @mixin --a {b: c}
+  |        ^^^
+  '
+
+<===>
+================================================================================
+<===> double_underscore_name/input.scss
+@mixin __a() {b: c}
+d {@include __a}
+
+<===> double_underscore_name/output.css
+d {
+  b: c;
+}
+
+<===>
+================================================================================
+<===> custom_ident_include/input.scss
+@mixin __a() {b: c}
+d {@include --a}
+
+<===> custom_ident_include/output.css
+d {
+  b: c;
+}
+
+<===> custom_ident_include/warning
+DEPRECATION WARNING: Sass @mixin names beginning with -- are deprecated for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+
+  ,
+2 | d {@include --a}
+  |             ^^^
+  '
+    input.scss 2:13  root stylesheet
diff --git a/spec/libsass/selectors/simple.hrx b/spec/libsass/selectors/simple.hrx
index 706b807c7d..b87ebeb52e 100644
--- a/spec/libsass/selectors/simple.hrx
+++ b/spec/libsass/selectors/simple.hrx
@@ -14,9 +14,6 @@ div {
 @-webkit-keyframes {
   from {
     left: 0px;
-    10% {
-      whatever: hoo;
-    }
   }
   to {
     left: 200px;
@@ -57,9 +54,6 @@ div:nth(-3) {
 @-webkit-keyframes {
   from {
     left: 0px;
-    10% {
-      whatever: hoo;
-    }
   }
   to {
     left: 200px;