I am not sure why it did work, but this is not exactly how it should be used. Let me explain.

Both [[size]] and [[span]] tags create inline elements, while [[collapsible]] creates a block element. In valid HTML it is not correct to embed block elements within inline elements. Moreover, the example you posted interleaves both tags (span opens, collapsible opens, span closes, collapsible closes), which creates invalid HTML. Since we are using some techniques to fix invalid HTML, sometimes such combination might lead to working results, but this is not guaranteed.

Solution 1

[[module CSS]]
.block_1 .collapsible-block-link {
     font-size: 150%;
     font-weight: bold;
     font-style: italic;
}
[[/module]]
[[div class="block_1"]]
[[collapsible show="Show This Block" hide="Hide This Block"]]
Some text to show/hide
[[/collapsible]]
[[/div]]

This way you can style each individual block, wrapping them in divs with different class names and using this class name in CSS rules.

See this page: http://try.wikidot.com/collapsible-styling for a working test case.

Solution 2 (ugly)

No, it is too ugly to even write it down.

I hope this helps. Again, I am not sure why it did work, but it was definitely not error-safe.

OK, so after looking into our code we have found a number of small improvements that can be applied to how we handle collapsible blocks.

Leaving the technical details aside, your styling should work again. However, this is not a recommended usage and I cannot guarantee it will work in the future.

See a test case at: http://try.wikidot.com/collapsible-styling

I hope this helps!