Is <img> element block level or inline level?

An img element is a replaced inline element.

It behaves like an inline element (because it is), but some generalizations about inline elements do not apply to img elements.

e.g.

Generalization: "Width does not apply to inline elements"

What the spec actually says: "Applies to: all elements but non-replaced inline elements, table rows, and row groups "

Since an image is a replaced inline element, it does apply.

It's true, they are both - or more precisely, they are "inline block" elements. This means that they flow inline like text, but also have a width and height like block elements.

In CSS, you can set an element to display: inline-block to make it replicate the behaviour of images*.

Images and objects are also known as "replaced" elements, since they do not have content per se, the element is essentially replaced by binary data.

* Note that browsers technically use display: inline (as seen in the developer tools) but they are giving special treatment to images. They still follow all traits of inline-block.

For almost all purposes think of them as an inline element with a width set. Basically you are free to dictate how you would like images to display using CSS. I generally set a few image classes like so:

img.center {display:block;margin:0 auto;}

img.left {float:left;margin-right:10px;}

img.right  {float:right;margin-left:10px;}

img.border  {border:1px solid #333;}

IMG elements are inline, meaning that unless they are floated they will flow horizontally with text and other inline elements.

They are "block" elements in that they have a width and a height. But they behave more like "inline-block" in that respect.