rerenders() injects an adopted stylesheet that flashes every element
with a blue background whenever it rerenders — a drop-in visualiser
for render thrashing. The flash is a one-second CSS animation that
runs each time an element is (re)created and enters the DOM; the
stylesheet is adopted on document, so call it once and watch the
page. Part of pota/use/test.
Takes no arguments.
Returns: nothing — the stylesheet stays adopted on document.