As described in this comment. In order to make an image's sizes value more accurately reflect the constraints of its alignment container, we need to incorporate the way padding settings affect the width of the container. For example, in the Twenty Twenty-four theme, the default settings.layout.contentSize setting is 620px, which is the max size that the container will be on larger viewport widths, but that width gets constrained by the padding styles for the site.

However, in many cases, theme.json padding styles reference the global padding settings, using CSS variables like var(--wp--preset--spacing--50), which cannot be calculated by the browser when the sizes attribute is parsed, so we will need a way to parse these settings into valid media query values.