How to apply a HTML tag to the entire post and type it only once?

I read How do I use a small font size in questions and answers? and Allow the tag, but I don’t want to write for each paragraph. For example, I want everything below the line to be in a small font, but it fails. First, A fails to be quoted in yellow. Second, neither A nor B are smaller. and also I see a lot of the designing font style on the Anchor Fonts website Now I want to use this designing font style in this tag