diff --git a/crates/oxc_span/src/source_type/mod.rs b/crates/oxc_span/src/source_type/mod.rs
index f58acb3a80b92..ee22fd1fa72a7 100644
--- a/crates/oxc_span/src/source_type/mod.rs
+++ b/crates/oxc_span/src/source_type/mod.rs
@@ -468,25 +468,35 @@ impl SourceType {
                 )
             })?;
 
-        let (language, module_kind) = match extension {
-            "js" | "mjs" | "jsx" => (Language::JavaScript, ModuleKind::Module),
-            "cjs" => (Language::JavaScript, ModuleKind::Script),
-            "ts" if file_name.ends_with(".d.ts") => {
-                (Language::TypeScriptDefinition, ModuleKind::Module)
-            }
-            "mts" if file_name.ends_with(".d.mts") => {
-                (Language::TypeScriptDefinition, ModuleKind::Module)
+        let module_kind = match extension {
+            "js" | "tsx" | "ts" | "jsx" | "mts" | "mjs" => ModuleKind::Module,
+            "cjs" | "cts" => ModuleKind::Script,
+            _ => unreachable!(),
+        };
+
+        let language = match extension {
+            // https://www.typescriptlang.org/tsconfig/#allowArbitraryExtensions
+            // `{file basename}.d.{extension}.ts`
+            // https://github.com/microsoft/TypeScript/issues/50133
+            "ts" => {
+                if file_name[..file_name.len() - 3].split('.').rev().take(2).any(|c| c == "d") {
+                    Language::TypeScriptDefinition
+                } else {
+                    Language::TypeScript
+                }
             }
-            "cts" if file_name.ends_with(".d.cts") => {
-                (Language::TypeScriptDefinition, ModuleKind::Script)
+            "js" | "cjs" | "mjs" | "jsx" => Language::JavaScript,
+            "tsx" => Language::TypeScript,
+            #[allow(clippy::case_sensitive_file_extension_comparisons)]
+            "mts" | "cts" => {
+                if file_name[..file_name.len() - 4].ends_with(".d") {
+                    Language::TypeScriptDefinition
+                } else {
+                    Language::TypeScript
+                }
             }
-            "ts" | "mts" | "tsx" => (Language::TypeScript, ModuleKind::Module),
-            "cts" => (Language::TypeScript, ModuleKind::Script),
             _ => {
-                #[cfg(debug_assertions)]
                 unreachable!();
-                #[cfg(not(debug_assertions))]
-                return Err(UnknownExtension(format!("Unknown extension: {extension}").into()));
             }
         };
 
@@ -547,14 +557,12 @@ mod tests {
     #[test]
     #[allow(clippy::similar_names)]
     fn test_d_ts_from_path() {
-        let dts = SourceType::from_path("foo.d.ts")
-            .expect("foo.d.ts should be a valid TypeScript definition file path.");
-        let dmts = SourceType::from_path("foo.d.mts")
-            .expect("foo.d.mts should be a valid TypeScript definition file path.");
-        let dcts = SourceType::from_path("foo.d.cts")
-            .expect("foo.d.cts should be a valid TypeScript definition file path.");
-
-        for ty in &[dts, dmts, dcts] {
+        let dts = SourceType::from_path("foo.d.ts").unwrap();
+        let dmts = SourceType::from_path("foo.d.mts").unwrap();
+        let dcts = SourceType::from_path("foo.d.cts").unwrap();
+        let arbitrary = SourceType::from_path("foo.d.ext.ts").unwrap();
+
+        for ty in &[dts, dmts, dcts, arbitrary] {
             assert!(ty.is_typescript());
             assert!(ty.is_typescript_definition());
             assert!(!ty.is_javascript());
diff --git a/tasks/coverage/snapshots/parser_typescript.snap b/tasks/coverage/snapshots/parser_typescript.snap
index b4a45f93a6b50..9cbe6b1320bce 100644
--- a/tasks/coverage/snapshots/parser_typescript.snap
+++ b/tasks/coverage/snapshots/parser_typescript.snap
@@ -2,7 +2,7 @@ commit: d85767ab
 
 parser_typescript Summary:
 AST Parsed     : 6494/6503 (99.86%)
-Positive Passed: 6481/6503 (99.66%)
+Positive Passed: 6483/6503 (99.69%)
 Negative Passed: 1275/5747 (22.19%)
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/ClassDeclaration24.ts
 Expect Syntax Error: tasks/coverage/typescript/tests/cases/compiler/ExportAssignment7.ts
@@ -4765,24 +4765,6 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/moduleResolut
    ·     ▲
    ╰────
   help: Try insert a semicolon here
-Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlFileWithinDeclarationFile.ts
-
-  × Function implementation is missing or not immediately following the declaration.
-    ╭─[typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlFileWithinDeclarationFile.ts:11:5]
- 10 │ export class HTML5Element extends HTMLElement {
- 11 │     connectedCallback(): void;
-    ·     ─────────────────
- 12 │ }
-    ╰────
-Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlImport.ts
-
-  × Function implementation is missing or not immediately following the declaration.
-    ╭─[typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlImport.ts:11:5]
- 10 │ export class HTML5Element extends HTMLElement {
- 11 │     connectedCallback(): void;
-    ·     ─────────────────
- 12 │ }
-    ╰────
 Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/salsa/plainJSRedeclare3.ts
 
   × Identifier `orbitol` has already been declared
diff --git a/tasks/coverage/snapshots/semantic_typescript.snap b/tasks/coverage/snapshots/semantic_typescript.snap
index d87e4bac54437..10f3a3f22ab56 100644
--- a/tasks/coverage/snapshots/semantic_typescript.snap
+++ b/tasks/coverage/snapshots/semantic_typescript.snap
@@ -2,7 +2,7 @@ commit: d85767ab
 
 semantic_typescript Summary:
 AST Parsed     : 6503/6503 (100.00%)
-Positive Passed: 2848/6503 (43.80%)
+Positive Passed: 2852/6503 (43.86%)
 tasks/coverage/typescript/tests/cases/compiler/2dArrays.ts
 semantic error: Symbol reference IDs mismatch for "Cell":
 after transform: SymbolId(0): [ReferenceId(1)]
@@ -8118,11 +8118,6 @@ semantic error: Scope children mismatch:
 after transform: ScopeId(0): [ScopeId(1), ScopeId(2)]
 rebuilt        : ScopeId(0): [ScopeId(1)]
 
-tasks/coverage/typescript/tests/cases/compiler/declarationEmitTransitiveImportOfHtmlDeclarationItem.ts
-semantic error: Scope children mismatch:
-after transform: ScopeId(0): [ScopeId(1)]
-rebuilt        : ScopeId(0): []
-
 tasks/coverage/typescript/tests/cases/compiler/declarationEmitTripleSlashReferenceAmbientModule.ts
 semantic error: Unresolved references mismatch:
 after transform: ["Url"]
@@ -20383,15 +20378,12 @@ rebuilt        : ScopeId(0): [ScopeId(1), ScopeId(2)]
 
 tasks/coverage/typescript/tests/cases/compiler/moduleAugmentationsImports1.ts
 semantic error: Bindings mismatch:
-after transform: ScopeId(0): ["./a", "A", "B", "Cls"]
-rebuilt        : ScopeId(0): ["A"]
-Scope children mismatch:
-after transform: ScopeId(0): [ScopeId(1), ScopeId(2), ScopeId(3), ScopeId(6)]
-rebuilt        : ScopeId(0): [ScopeId(1), ScopeId(2)]
+after transform: ScopeId(0): ["A", "a", "b", "c"]
+rebuilt        : ScopeId(0): ["a", "b", "c"]
 
 tasks/coverage/typescript/tests/cases/compiler/moduleAugmentationsImports2.ts
 semantic error: Bindings mismatch:
-after transform: ScopeId(0): ["./a", "A", "B"]
+after transform: ScopeId(0): ["./a", "A", "Cls"]
 rebuilt        : ScopeId(0): ["A"]
 Scope children mismatch:
 after transform: ScopeId(0): [ScopeId(1), ScopeId(2)]
@@ -46349,30 +46341,18 @@ semantic error: Unresolved references mismatch:
 after transform: ["const"]
 rebuilt        : []
 
-tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlFileWithinDeclarationFile.ts
-semantic error: Function implementation is missing or not immediately following the declaration.
-
-tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFileForHtmlImport.ts
-semantic error: Function implementation is missing or not immediately following the declaration.
-
-tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFileForJsonImport.ts
-semantic error: Bindings mismatch:
-after transform: ScopeId(0): ["val"]
-rebuilt        : ScopeId(0): []
-Reference symbol mismatch for "val":
-after transform: SymbolId(0) "val"
-rebuilt        : <None>
-Reference flags mismatch for "val":
-after transform: ReferenceId(0): ReferenceFlags(Read)
-rebuilt        : ReferenceId(0): ReferenceFlags(Read | Type)
+tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFilesForNodeNativeModules.ts
+semantic error: Missing SymbolId: "mod"
+Missing ReferenceId: "require"
+Binding symbols mismatch:
+after transform: ScopeId(0): [SymbolId(0)]
+rebuilt        : ScopeId(0): [SymbolId(0)]
+Reference symbol mismatch for "mod":
+after transform: SymbolId(0) "mod"
+rebuilt        : SymbolId(0) "mod"
 Unresolved references mismatch:
 after transform: []
-rebuilt        : ["val"]
-
-tasks/coverage/typescript/tests/cases/conformance/nonjsExtensions/declarationFilesForNodeNativeModules.ts
-semantic error: Scope children mismatch:
-after transform: ScopeId(0): [ScopeId(1)]
-rebuilt        : ScopeId(0): []
+rebuilt        : ["require"]
 
 tasks/coverage/typescript/tests/cases/conformance/override/override10.ts
 semantic error: Scope children mismatch: