The width & height attributes

The height and width attributes tell the browser the exact size of the image in pixels (px) and speed up the downloading process.

The browser first loads the text and then the image files. If the browser knows the dimensions of the image, it can set the space size aside while laying out the page. If you don't indicate the image's dimensions, the page's layout will shift as it loads. As you can probably guess, this is extremely disruptive to users — especially when they have already started reading the article! Another downside to not indicating the image height and width is that browsers might stretch or reduce them as they will.