? Crazy?
Things could get… ugly, heh. The last thing I need to see is bright pink, bold, size 72 text against a lime green background when I load a song. Text formatting is a privilege, not a right! 
But seriously, I don’t think an HTML engine is necessary. It’s just a little comments box afterall, not a word processor. I think maybe some formatting options might be interesting. Just simple stuff like bold, italics, underline. They would provide enough visual enhancement to your comments without being too distracting I think.
Thinking about it a little more though, what might be interesting is if the comments box supported basic ANSI formatting. Then you kinda have the best of both worlds - the simplicity of it all being one, uniform typeface, with the extra formatting that ANSI provides for anyone who wants to use it. I don’t think Renoise’s interface should necessarily have all these complex options for composing nicely formatted comments, but perhaps it could retain the formatting when copy/pasting the text in from another editor.
Of course, things could just as easily get ugly with ANSI abuse, so this is neither here nor there I guess. But if I had to choose between HTML or the more traditional ANSI style, I’d go for the ANSI.
At the moment though I’m quite happy with with the monospace ASCII we currently have… it’s quite sufficient for most needs.
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If it was something that suddenly just appears and could be blaring and distracting, then I would say no. But I could maybe see it if it was a separate window (like a moveable pop-up window on top of Renoise) that opened to see the HTML part if you click on a summary, and does not appear at any other time.
It wouldn’t be useful to me, but that’s how I think now. I may not find it useful, but someone else might.
Oh hell, I forgot about formatting… I was initially thinking just a hyperlink to a website or online mp3. That’s all. Goodness gracious no text formatting please~!
Lol, in the future i need a Pop-Up blocker for renoise ? 
So, please don’t forget the “Ignore all” Button to disable flash & java. 
But simple hyperlinks is a cool Idea.
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I think a more strict parsing would be better, like automatically turning URLs into links. Any kind of markup / styling just complicates an otherwise straightforward field.
$0.02. My, that is. 
I think it’s possible to put any file inside *.xrns as it’s a zip archive. So, you can attach a bunch of photos or promo html page to your song. But, it would be nice if Renoise could show the attached files/folders and could handle them somehow. For example, some “attached files” popup where you could click to the links and open the files with external application. It would also great if Renoise could decode some simple txt/url/nfo/jpg files. But it’s a general idea of mine.
Yeah I like that idea.
Late reply to this post, but I like the idea of a JPeG pop-up / splash screen of a fixed size to save in a song.
This way, everyone is happy about mark-up etc.
But I like it the most then for expressing your song with a “cover”.