New functionality is typically implemented with first class objects that import behavior from issues via shared concerns.

They cause completely different behavior for auto margins. If you have a fixed element for example with top/bottom/left/right set to zero and you stick an image in it you want to center wrapped in a div, then in order to center that div with auto margins, you MUST specify a CSS width/height, because specifying an HTML attribute width/height has no effect and the margins remain zero. I have no idea why the difference exists.

Whether to specify in html or css is best judged on individual circumstances. A large number of images of the same size would probably be best served with css, a single image with html. That said, if you are specifying other styles for the image (border colour, style or radius, float etc) it would make sense to add width & height to the css.