just move the style instead

lib/elements/custom-style.html

@@ -96,19 +96,13 @@
       https://crbug.com/523952
 
       If this element is not in the main document, then it must be in an HTML Import document.
-      In that case, copy the style into the main document and disable the current style.
-      `__appliedElement` is used by the HTML Imports polyfill for this use case, and already understood by
-      ShadyCSS's Custom Style Interface.
+      In that case, move the custom style to the main document.
 
       The ordering of `<custom-style>` should stay the same as when loaded by HTML Imports, but there may be odd
       cases of ordering w.r.t the main document styles.
       */
       if (this.ownerDocument !== window.document) {
-        let appliedStyle = style.cloneNode(true);
-        style.__appliedElement = appliedStyle;
-        // disable style in the HTML Import document
-        style.setAttribute('media', 'none');
-        window.document.head.appendChild(appliedStyle);
+        window.document.head.appendChild(this);
       }
       return this._style;
     }