These elements both have a function outside of the visual design representation: they provide information for other user-agents to help communicate the information more effectively.

That’s part of the illusion of design: people frequently believe that they’re adding paragraph tags (or emphasis, etc.) in order to create a visually separated block of text, when actually the purpose of the element is to create a paragraph, regardless of the visual presentation of that text. It’s still machine-readable as a paragraph even if the designer has (foolishly) chosen to collapse the margins and padding on all paragraphs. The visual designer has broken the paradigm of a paragraph for visual users, but screen readers will still gain the benefit.

Thanks for your comments, Jennifer!

Why? Because whenever you try to get someone to post content sans design elements, they can’t. For example, even here, I’ve added paragraph spaces (assuming they show up in the comment), em tags for emphasis which I’m hoping will show up as italics, and so on.

When I worked as a CMS designer, I was always striving to find the “perfect” input form. This would be the form that was easy to use, but took all the design away from the writers. In one horrible iteration, we had an article form that was built one paragraph at a time. And my content authors would paste in entire Word docs into the first paragraph field. When we explained they had to do it paragraph by paragraph, they rebelled. So in that instance, we had created a fully separated content/design system. But the people involved wouldn’t use it.

The use of freely-available themes (such as with Wordpress) does lead to a lot of low-quality code — but the greater concern is professionals who aren’t paying close attention to what they send out.