One of the most important concepts in AMP’s design is its focus on reducing the amount of DOM reflow required to render its web pages. To reduce DOM reflow, AMP includes a layout system to ensure the layout of the page is known as early as possible in the lifecycle of downloading and rendering the page.

Why was the container type inferred? Because we did not specify a height attribute for the amp-img tag. In HTML, reflow can be reduced by always specifying a fixed width and height for elements on a page. In AMP, you need to define the width and height for amp-img elements so that AMP can pre-determine the aspect ratio of the element.