If we want to actually start using PHP OOUI for basic forms in MediaWiki core, we'll presumably need a SVG+PNG distribution that would serve SVG images to modern browsers and PNG as a fallback (probably using the same technique we use for this in MediaWiki). This would be an addition to the existing two distributions, PNG-only (raster) and SVG-only (vector).

We are currently using the SVG (vector) distribution of OOUI in MediaWiki, since we were only used it for advanced JavaScript interfaces (like VisualEditor), only supported by browsers that support SVG anyway. But now we'll need the images for basic features done in PHP (like the new MediaWiki theme checkbox widgets), since we still support browsers such as IE 8 (list), which have no SVG support.