Knowing HTML is understanding document structure: That return to the roots of the language means you can focus your HTML learning on the tags and styles that explain document structure -- paragraphs, lists, headings, sections, images, captions. You know, the content types you work with every day.
HTML logic is quite simple, and the markup quickly makes sense to writers and editors, in particular -- because the language is set up much the way you have to think to put one word in front of another competently.