Element.slot does not ensure Element is slotted

Always read the spec, not MDN.

Author: cdoublev

lib/cssom/CSSStyleDeclaration-impl.js

@@ -6,11 +6,33 @@ import { isFailure, isOmitted } from '../utils/value.js'
 import { serializeDeclarationBlock, serializeValue } from '../serialize.js'
 import expandShorthandDeclaration from '../parse/shorthand.js'
 import { getResolvedValue } from '../resolve.js'
+import { isOfType } from '../utils/dom/element.js'
+import { iterate } from '../utils/dom/collection.js'
 import logical from '../properties/logical.js'
 import properties from '../properties/definitions.js'
 import shorthands from '../properties/shorthands.js'
 import { toLowerCase } from '../utils/string.js'
 
+/**
+ * @param {Element} element
+ * @returns {boolean}
+ */
+function hasResolvedStyle(element) {
+    if (!element.isConnected) {
+        return false
+    }
+    const shadowRoot = element.parentElement?.shadowRoot
+    if (shadowRoot) {
+        for (const node of iterate(shadowRoot.childNodes)) {
+            if (isOfType(node, 'slot') && node.assignedNodes().some(slotted => slotted === element)) {
+                return true
+            }
+        }
+        return false
+    }
+    return true
+}
+
 /**
  * @param {object} declaration
  * @param {object[]} declarations
@@ -173,7 +195,7 @@ export default class CSSStyleDeclarationImpl {
             return ''
         }
         if (this._computed) {
-            if (!this._ownerNode.isConnected || (!this._ownerNode.slot && this._ownerNode.parentElement?.shadowRoot)) {
+            if (!hasResolvedStyle(this._ownerNode)) {
                 return ''
             }
             if (shorthands.has(name)) {