There's just one problem with having no width and height specified for an img tag, and that's what happens if the image can't
be loaded for some reason. If the img tag has a width and height, the page layout still leaves the same space for the image, and displays the alt text, instead. If there are no
width and height specified, and the image can't be loaded, the boxes for the now-missing images are resized to fit the alt text, instead. In some cases, that can louse up a page layout.
So, while width and height aren't required by the specs, I think it's safer to use them. But not in a "style" tag, because of the nasty things that does to poor Highslide JS!