Wikipedia used to use spoiler alerts that you'd have to unfold, and then some idiots went and argued for removing all the spoiler warnings on some idiotic grounds*.
I never look at the summaries any more unless I really do want to know and don't intend to actually read/watch the work in question.
* "It's pop culture, so it's common knowledge, you wouldn't look it up in an encyclopedia unless you wanted to know, it's your fault for even looking, plot-ruining information wants to be freeeeeeeeeeee!" is how I think I recall it going.