Broken references in Portals #285

While crawling Portals, the following links to other specifications were detected as pointing to non-existing anchors, which should be fixed:

• https://html.spec.whatwg.org/multipage/web-messaging.html#postmessageoptions
• https://html.spec.whatwg.org/multipage/web-messaging.html#dom-postmessageoptions-transfer

_{This issue was detected and reported semi-automatically by Strudy based on data collected in webref.}