If you're composing a root node like this one, and you use <spoiler> ... </spoiler> tags when you hit preview you'll see 
[ ] Preview spoil [ preview ][ create ]

[download]

Checking the checkbox and pressing preview, unhides the content in spoiler tags

That really ought to work for replies

In reply to spoiler tags "Preview spoil" not available for replies by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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