You're not a dummy. I was making same mistakes when I just started to play in html. Actually, it's not mistake, but rather, you are paying attention to other things while you are constructing your html page, and not paying to much attention on form of the paragraphs, pictures, and animations...
You'll get used to it, just wait... one day, you'll start noticing more and more things, and you'll say: "...oh, why did I do this like that, and not like this ? This is much better and easier way..."
Cheers !