"Also, long blocks of code are not semantically spoilers."
That is an additional use for them. In "Writeup Formatting Tips", you'll see:
"If you want to obscure some text for some reason (like the punchline for a joke, or the answer to a tricky question) then you should use <spoiler> tags."
Furthermore, given you've brought up semantics, the word "details" is hardly semantically correct for an element which, by default, does not show details.
"... it is also more correct to use the proper HTML tag."
No, that's just wrong. In "Markup in the Monastery", you'll see:
"PM markup code is NOT entirely w3c html 4.01 standard nor is it XHTML, so even if you're expert please scan these notes."
Reading the notes that follow will show it's NOT any other HTML version either. There's also more information about spoilers.
"It might be a good idea to drop <font> as one day browsers might drop it."
In "Writeup Formatting Tips", you'll find:
"Almost any use of <font> tags will be wrong for at least some users, so you shouldn't use them. At all. The only reason <font> is allowed is for backwards compatibility."
[As I've included quotes from multiple sources, I've added 'You wrote:' in multiple places to separate the parts of your reply to which I was responding. In the initial preview without those, I thought there might be some confusion. Except as an aid to sectioning, the emboldening is not significant.]