Add resolveProperty callback option

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: domenic

lib/CSSStyleDeclaration.js

@@ -27,7 +27,7 @@ class CSSStyleDeclaration {
    * @param {object} [opt] - Options.
    * @param {object} [opt.context] - The context object (Window, Element, or CSSRule).
    */
-  constructor(onChangeCallback, { context } = {}) {
+  constructor(onChangeCallback, { context, resolveProperty } = {}) {
     // Internals for jsdom
     this._global = globalThis;
     this._onChange = onChangeCallback;
@@ -40,6 +40,10 @@ class CSSStyleDeclaration {
     this._readonly = false;
     this._updating = false;
 
+    // Lazy resolution callback
+    this._resolveProperty = resolveProperty || null;
+    this._resolvedValues = resolveProperty ? new Map() : null;
+
     // Other internals
     this._length = 0;
     this._propertyIndices = new Map();
@@ -243,6 +247,16 @@ class CSSStyleDeclaration {
    * @returns {string} The property value, or empty string if not set.
    */
   getPropertyValue(property) {
+    if (this._resolveProperty && this._readonly) {
+      if (this._resolvedValues.has(property)) {
+        return this._resolvedValues.get(property);
+      }
+      const resolved = this._resolveProperty(property);
+      if (resolved !== undefined) {
+        this._resolvedValues.set(property, resolved);
+        return resolved;
+      }
+    }
     if (this._values.has(property)) {
       return this._values.get(property).toString();
     }