HTML documents don't generally have the concept of lines since text reflow based on the way user agent (browser) is rendering the document. That's why the RFC only talks about text/plain MIME type - browsers render those with all line breaks intact. You can maybe use a CSS selectors, but that adds a tremendous amount of complexity and is not easy for non-web developers to read anymore.

Oops, good point. I suppose you could do by newline/paragraph plus character instead, but that would be even more vulnerable to text changes.