From 9aba9cbe003effb5ea61466bc50c3d956472b980 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 02:09:54 +0000
Subject: [PATCH 1/2] Initial plan


From 25fbefe68efe54a8f7faae6a9ffaf009f867ed85 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 02:25:16 +0000
Subject: [PATCH 2/2] Allow kebab-case indexing on CSSStyleDeclaration

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
---
 tsc/internal/bundled/libs/lib.dom.d.ts        |  4 ++++
 .../cssStyleDeclarationKebabCaseIndex.js      | 13 +++++++++++
 .../cssStyleDeclarationKebabCaseIndex.symbols | 17 ++++++++++++++
 .../cssStyleDeclarationKebabCaseIndex.types   | 23 +++++++++++++++++++
 .../cssStyleDeclarationKebabCaseIndex.ts      |  7 ++++++
 5 files changed, 64 insertions(+)
 create mode 100644 tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.js
 create mode 100644 tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.symbols
 create mode 100644 tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.types
 create mode 100644 tsc/testdata/tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts

diff --git a/tsc/internal/bundled/libs/lib.dom.d.ts b/tsc/internal/bundled/libs/lib.dom.d.ts
index 9e127c3f5a524..b6a3f1c8f43ad 100644
--- a/tsc/internal/bundled/libs/lib.dom.d.ts
+++ b/tsc/internal/bundled/libs/lib.dom.d.ts
@@ -6587,6 +6587,10 @@ interface CSSStyleDeclarationBase {
      */
     setProperty(property: string, value: string | null, priority?: string): void;
     [index: number]: string;
+    /**
+     * CSS properties can also be accessed using their kebab-case (dashed) names, e.g. `style['background-color']`.
+     */
+    [index: string]: any;
 }
 
 interface CSSStyleDeclaration extends CSSStyleProperties {
diff --git a/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.js b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.js
new file mode 100644
index 0000000000000..9bc40fe74b797
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.js
@@ -0,0 +1,13 @@
+//// [tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts] ////
+
+//// [cssStyleDeclarationKebabCaseIndex.ts]
+declare const el: HTMLElement;
+
+const value = getComputedStyle(el)["background-color"];
+el.style["background-color"] = "red";
+
+
+//// [cssStyleDeclarationKebabCaseIndex.js]
+"use strict";
+const value = getComputedStyle(el)["background-color"];
+el.style["background-color"] = "red";
diff --git a/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.symbols b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.symbols
new file mode 100644
index 0000000000000..9baac0f1a4c04
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.symbols
@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts] ////
+
+=== cssStyleDeclarationKebabCaseIndex.ts ===
+declare const el: HTMLElement;
+>el : Symbol(el, Decl(cssStyleDeclarationKebabCaseIndex.ts, 0, 13))
+>HTMLElement : Symbol(HTMLElement, Decl(lib.dom.d.ts, --, --), Decl(lib.dom.d.ts, --, --))
+
+const value = getComputedStyle(el)["background-color"];
+>value : Symbol(value, Decl(cssStyleDeclarationKebabCaseIndex.ts, 2, 5))
+>getComputedStyle : Symbol(getComputedStyle, Decl(lib.dom.d.ts, --, --))
+>el : Symbol(el, Decl(cssStyleDeclarationKebabCaseIndex.ts, 0, 13))
+
+el.style["background-color"] = "red";
+>el.style : Symbol(ElementCSSInlineStyle.style, Decl(lib.dom.d.ts, --, --), Decl(lib.dom.d.ts, --, --))
+>el : Symbol(el, Decl(cssStyleDeclarationKebabCaseIndex.ts, 0, 13))
+>style : Symbol(ElementCSSInlineStyle.style, Decl(lib.dom.d.ts, --, --), Decl(lib.dom.d.ts, --, --))
+
diff --git a/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.types b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.types
new file mode 100644
index 0000000000000..d8e3f4c36e5bc
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/cssStyleDeclarationKebabCaseIndex.types
@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts] ////
+
+=== cssStyleDeclarationKebabCaseIndex.ts ===
+declare const el: HTMLElement;
+>el : HTMLElement
+
+const value = getComputedStyle(el)["background-color"];
+>value : any
+>getComputedStyle(el)["background-color"] : any
+>getComputedStyle(el) : CSSStyleDeclaration
+>getComputedStyle : (elt: Element, pseudoElt?: string | null) => CSSStyleDeclaration
+>el : HTMLElement
+>"background-color" : "background-color"
+
+el.style["background-color"] = "red";
+>el.style["background-color"] = "red" : "red"
+>el.style["background-color"] : any
+>el.style : CSSStyleDeclaration
+>el : HTMLElement
+>style : CSSStyleDeclaration
+>"background-color" : "background-color"
+>"red" : "red"
+
diff --git a/tsc/testdata/tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts b/tsc/testdata/tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts
new file mode 100644
index 0000000000000..3c0b4e1c02b6a
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/cssStyleDeclarationKebabCaseIndex.ts
@@ -0,0 +1,7 @@
+// @lib: dom,es2015
+// @noImplicitAny: true
+
+declare const el: HTMLElement;
+
+const value = getComputedStyle(el)["background-color"];
+el.style["background-color"] = "red";