To give a concrete example: Wiki-hosted code SHOULD NOT manually replicate the HTML markup of a dropdown menu in Vector, using Vector's stylesheets as the gadget incorrectly assumes the HTML markup of the dropdown menu is stable.
The sentence structure here seems slightly confusing and I don't think it really explains what the issue is, perhaps rewording to "Wiki-hosted code SHOULD NOT manually replicate the HTML markup of a dropdown menu in Vector, using Vector's stylesheets, as both the html structure of the menu and the content of the stylesheet are considered unstable and may change" would make the problem clearer?
Perhaps it would also be worth adding a pointer to what developers should do instead? e.g. "Instead wiki-hosted code should use the addPortletLink function to add additional menu items, because as part of the mw object it would be considered stable."