No optimization is intrinsically premature. Most of these are not things you need to worry about, but there will be cases where it makes a different. That said

No optimization is intrinsically premature. Most of these are not things you need to worry about, but there will be cases where it makes a different. That said, specifying image sizes is often a good idea in order to prevent the page from moving around while it loads. It looks a lot cleaner if your text all flows correctly and then the images pop into place.