You can't use an SVG as an image in a notification. A real shame as it would have solved all the device pixel ratio problems, but the SVG spec is absolutely huge (and includes stuff like animation) so I'm not overly surprised that Android doesn't support it.
Hmm, I'm wondering if there's a way now to render an SVG (not the full spec) within a service worker e.g. using PngPong with https://github.com/canvg/canvg
Huh! That's a really fascinating library. My gut instinct is still that it would be a heavy lift, though. Without the Canvas API in the middle you'd need to reimplement pixel by pixel drawing of different shape/curves (PngPong literally only draws squares and copies sections of other images) anti-aliasing, etc. etc.
It would be a fascinating thing to work on, but the OffscreenCanvas API is coming to Chrome[1], so I suspect it would end up being a lot of work that would be obselete after not all that much time.
