Spoilers don't work properly.

example spoiler

It looks like I'm just quoting myself! I don't want to look like you-know-who, so it'd be good if you could add the CSS to match the syntax.

.md-spoiler-text * {
    visibility: hidden;
}

.md-spoiler-text:hover *, .md-spoiler-text:focus * {
    visibility: visible;
}

On Reddit there's some JavaScript causing it to appear on clicking, probably because there's CSS like this:

.md-spoiler-text.md-spoiler-shown * {
    visibility: visible;
}

and then some JavaScript to set / toggle the class on click of the element.