“Does the media query match” is certainly the simplest way to defer loading, but I think some leeway could be useful; the user agent may have ways of predicting when there’s a reasonable probability that the media query will match soon. Kinda like how <img loading=lazy> doesn’t load once the image is needed, but a short time before it’s probably needed.

Hmm, thinking about <img loading=lazy>, I’ve thought of a reason they might not load such stylesheets lazily: privacy leak, you’re notifying the server that the document may have been printed. Lazy image loading has been careful to address this situation so that it’s no new surface (e.g. browsers shouldn’t lazy load if JS is disabled, but if it’s enabled you’re not leaking anything new). But printing specifically? I guess there hasn’t been any way of detecting that the document has been printed, so doing lazy loading on stylesheets for different media types would reveal something new.