Several small changes are being made to edge cases of the selectedcontent element: - When multiple selectedcontent elements are put in a select element at the same time, all of them will be kept up to date instead of only the first one in DOM order. - Updating the selectedcontent element is deferred when it would run during insertion, removal, or moving steps in order to fix security issues. The update is deferred by using post-insertion steps or microtasks. This is a PSA instead of an intent to ship because this only changes invalid cases or edge cases, and the usage of customizable select is still quite low (0.02%).