Jay R. Ashworth wrote:
*How* the poem tag does what it does is precisely the way you, I, and everyone else who I've seen post thinks it ought to: by leveraging CSS.
Actually, based on Brion's original description, it does rather more than that. In fact, it introduces _new syntax_ that applies to any wikitext enclosed in it.
Specifically, the <poem> environment introduces one new syntax element (the linefeed: normally ignored when not followed by other special markup, now creates a line break in output) and modifies the meaning of another (leading whitespace: normally marks a verbatim preformatted paragraph, now produces indentation).
Consider the fact that any existing poem on a MediaWiki page would break horribly if wrapped in <poem> tags without changing the markup to match the new syntax. This is quite different from your typical semantic HTML tag, like <code> or <blockquote>, which can generally be wrapped around an existing block of markup without changing the way it is parsed. In this respect the <poem> tag is more like <pre> or even <table>, both of which are unique presentational tags that require unusual markup to be used within them.