You can comment using an account on any ActivityPub-compatible instance. On most platforms, you can find the video by typing its URL in the search bar and then comment it from within the software's interface.
Tags could be used to suggest relevant recommendations. <x id="LINE_BREAK"/> There is a maximum of 5 tags. <x id="LINE_BREAK"/> Press <x id="START_TAG_KBD"/>Enter<x id="CLOSE_TAG_KBD"/> to add a new tag.
A timestamp (<x id="START_ITALIC_TEXT" ctype="x-i" equiv-text="<i>"/>00:05<x id="CLOSE_ITALIC_TEXT" ctype="x-i" equiv-text="</i>"/> for example) is automatically converted into a link to a part of the video.