The target selector for the rendered fragments.
When the given target contains an abstract selector like :main or :layer,
this property describes the specific selector that matched:
let result = await up.render({ target: ':main' })
result.renderOptions.target // result: ":main"
result.target // result: "[up-main='']"
If any [up-hungry] elements were added to the render pass, they
are not included in result.target.
