For the stand-alone WYSIWYG editor we could synchronize the HTML HEAD because we're using a separate DOM document, and thus we know that everything inside the HEAD of the edited document is related to the edited document. We can't do the same for the in-place editor though because the in-place editor is not using a separate document, so it shares the HEAD with everything on the page (like panels or UI extensions). We can imagine two users with different access rights editing the same page in-place. They may see different panels or have different UI extensions (based on their access rights or group membership) and these may require special CSS and JavaScript that we shouldn't synchronize because we could get unexpected side effects.

The only viable solution I have in mind is to not sync the macro output at all and to re-execute the macro for each co-editor, so that the required CSS and JavaScript are injected in the HEAD.