When I was browsing through a stackexchange site recently, I noticed this:
As you can see, the tags chemistry and mathematics were showing through the spoiler section. The cause for this is that the spoiler css masks the color property of the tag, but not the background-color, so the blue background bleeds through.
Here is an image of the css: (It's quite small so you'll have to open it in a new tab and zoom in).
As you can see from the strikethrough, the color is being overwritten by the spoiler but the tag is not.
Please fix this by changing the css for spoilers to overwrite background color!