Currently, the export feature (e.g. http://deckbox.org/sets/123/export) returns an unstyled list of card names, which is great for copy-pasting. But it returns this list as html, instead of text. This means that if you try to request the url programmatically or go to save the export from the browser you get all the unnecessary markup. Returning this as plaintext would solve both these problems.

Well, you can always do something along those lines...

https://jsfiddle.net/9s2m69bL/

And there may be thousand other ways to do it as well.

Sure, it just seems simpler on all fronts to get text in the first place, rather than making anything that wants to consume it strip out itself.

I just think, that there is so much to do for this website, that there will be no time to do this.
Maybe you should just live with it and do a small workaround for it.

Oh, no argument that there are more important things. But that doesn't make it an invalid point, and it may be something simple enough to fix quickly without much ado.