Spoiler text is invisible until the user mouses over the spoiler box, at which point it appears. This uses a fade-in animation, and there is likewise a fade-out animation when the cursor leaves the spoiler text box. In this answer on Code Golf, however, the answer includes this spoiler:
It just prints a
√character in front of the number entered.
There is a
<code> section in the spoiler, and this portion of the text does not fade in or out: it simply appears when the cursor enters the box and disappears when the cursor leaves it. This is a minor issue but I would regard it as a bug.