When you publish ASP.NET code from Tridion using the TCDL mechanism, it adds extra output that you didn't ask for. Specifically, each component presentation is emitted wrapped in a span tag. The span tag has an ID composed of the URI's of the component and component template.

These span tags aren't there because you asked for them in your content, or in your templates, so why are they there?

I reported this bug to Tridion customer support, and they told me that it's not a bug. It's designed that way, they say. Fair enough, I say, then it's a design bug. Apparently, the purpose is "to make it easier for SiteEdit and template designers to manipulate existing component presentations. Well that won't wash, because it's no earthly use for either purpose.

Is it a bug? Walks like a duck, quacks like a duck....

What harm can it do?

• Well in my case the main issue was that in XHTML a span tag isn't allowed to contain block-level elements. That means the XHTML produced this way is invalid. (I'm the kind of guy that likes to see a clean compile, and I like to see my web pages validate correctly too. This isn't just bravado, it's the most efficient way to work.)
  
• On top of that, you might not always be in charge of the CSS on your site. What if someone specifies a child selector instead of a descendant? That's just an example: you can imagine a hundred other ways this detritus could ruin your style.
  
• Oh heck! It's just broken, that's all. A web content management system should allow you to manage your web content. That means not emitting things you didn't ask for.
  

Anyway - what can you do about it?

• You can ignore it. This might be OK if you don't care about validation, and you have complete control of the styles.
  
• Of the people I've spoken to, it seems most are simply avoiding the problem by publishing to a Publishing Target with Target Language set to None. Of course, if you do this, you have to writeOut the various functions for linking etc. yourself. If everyone does this, then Tridion might as well never have invented TCDL in the first place.
  
• You can create a customised version of the AspDotNETTransformer. This is probably what I'll do. Tridion customer support have provided me with some java source code (which is just as well as it's an undocumented API). It looks straightforward enough and it's install once - fixed forever.