A good "what you see is what you get" editor is
Sigil. If you convert your html to epub using Calibre, you can edit the book to add the visual effects you want - italics, font-size, indented margins etc. - without bothering with the code (although you can edit the code as well if you like).
Personally I'm not a big fan of visually-formatted flashbacks unless they're very short - they can get annoying. If you're telling the story through the eyes of your characters, you might try setting your remembered sequences apart by having them in the present tense. It sounds counter-intuitive, but this can make them vivid and memory-like.
Or you could trust the reader to keep up. They're pretty smart.