HTML definitely sucks to parse but it's definitely better then plain text. At least it's a standardized format, as well that it allows for a much better interpretation in terms of visuals without convoluting the DOM for a parser. Sort of the best of both worlds, again I know using any HTML lib sucks to a degree, even with xpath, but wouldn't it still be a step forward.
Let's standardize all email error messages to HTML. At least then a standard exists even though it will suck, most standards suck in many ways. But it's a total nightmare without any.
No, no, no, a thousand times, no. HTML is not easier to parse than plain text. Plain text is easily transferred, stored, parsed (with grep, or whatever you use for regular expressions), and, as a bonus, is easy for humans to look at. HTML helps with exactly none of those things. There's a reason that Erik S. Raymond emphasizes the power of plain text in The Art of Unix Programming. There's a reason that The Pragmatic Programmer has a chapter devoted to it. That's because plain text (more specifically plain ASCII text) is the closest thing we have to a universally understood format in computing. For something like e-mail, plain text is by far preferable to HTML, XML, JSON or some other structured format. You can't know that your target will have a parser for a structured format. The only thing you can assume is that your target is capable of reading and transferring plain text.
Let's standardize all email error messages to HTML. At least then a standard exists even though it will suck, most standards suck in many ways. But it's a total nightmare without any.