[rerdavies]

Out of idle curiosity, which better element am I supposed to use when I'm using flex layout to space out elements on my page?

(e.g., a toolbar with with left- and right- justified elements, among which is an expanding searchbox).

[assimpleaspossi]

HTML elements are used to describe their content and not have anything to do with layout. While they often have common properties, these can be changed using CSS.

So use the element that best describes its content.