In Svelte, all reactive statements are memoized. Instead of const var = useMemo(() => expression, dependencies), you can use $: var = expression. Notice with Svelte, you don't need to declare the dependencies. The compiler infers them for you.

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.