On certain wikipedia's like zhwp, some users would put images or portal links, floating to the right side, in the References section with the single-column-by-default <references />. This becomes a concern when migration to responsive layout (T33597) is put on the table, as CSS multi-column layout tend to limit the whole div block inside a strict, rectangular box. As a result, an unfilled area appears below the flowed elements.
An obvious page-level fix is to move these things to a gallery or "see also" section, but it makes the config change objection-worthy. An alternative workaround on the CSS level would be doing clear: both to transfer the blame away from reference wrappers.