Originally, with html as it once was, I’d agree with that first part. Now with CSS, you can, if you try hard, do anything I’ve ever seen in a printed poem in an html/css page. But it is a long way from easy and it requires much more power than a typical forum will want to offer to the users.
For one famous example, consider “The Mouse’s Tail” from Alice’s Adventures: Lewis Carroll's "The Mouse's Tale"
Class is used for text size here, the style bits for positioning:
<div class="eleven" style="position:relative;left:-60px">"Fury said to</div>
<div class="ten" style="position:relative;left:-40px">a mouse, That</div>
<div class="ten" style="position:relative;left:0px">he met</div>
<div class="ten" style="position:relative;left:10px">in the</div>
<div class="ten" style="position:relative;left:20px">house,</div>
<div class="ten" style="position:relative;left:17px">'Let us</div>
<div class="ten" style="position:relative;left:5px">both go</div>
<div class="ten" style="position:relative;left:-7px">to law:</div>
<div class="ten" style="position:relative;left:-23px"><i>I</i> will</div>
<div class="ten" style="position:relative;left:-26px">prosecute</div>
<div class="nine" style="position:relative;left:-40px"><i>you.</i>—</div>
<div class="nine" style="position:relative;left:-30px">Come, I'll</div>
<div class="nine" style="position:relative;left:-20px">take no</div>
<div class="nine" style="position:relative;left:-7px">denial;</div>
<div class="nine" style="position:relative;left:19px">We must</div>
<div class="nine" style="position:relative;left:45px">have a</div>
<div class="nine" style="position:relative;left:67px">trial:</div>
And although I can’t find a live example, I can concoct one, “Jabberwocky”'s first stanza is typically shown mirrored in the book (Looking-Glass):
<style type=text/css>
.odd { position:relative }
.even { position:relative;left:1em }
.flip { transform: scale(-1, 1) }
</style>
<div class="odd flip">'Twas brillig, and the slithy toves</div>
<div class="even flip">Did gyre and gimble in the wabe;</div>
<div class="odd flip">All mimsy were the borogoves,</div>
<div class="even flip">And the mome raths outgrabe.</div>
There’s everything there to handle concrete poetry, Apollinaire’s “Calligrammes” (read “It’s Raining” if you never have), jumbled overlapped forms from experimental poetry, but all those are best done in isolated pages.
I’d start with the syntax highlighter, and see if that could be made to understand light markup while leaving whitespace unchanged.