For comparison, consider XML. XML’s specified behavior is to render nothing when the document contains unbalanced tags. Unbalanced XML tags are broken.
Unbalanced HTML tags render exactly the same way in all browsers. Unbalanced HTML tags are “invalid” but not broken.
P.S. I’ve discovered that I can use <blockquote> instead of [quote] and get back great-looking one-line quotations.
[quote]this is broken[/quote]
<blockquote>but this works great</blockquote>
but this works great
But I still don’t really care about [quote]. I care about spoilers.