> It's a class of problem that raster image formats basically don't have.

That took way too long to be this way. Some old browsers couldn't even get the colors of PNGs correct, let alone the transparency.

I would have expected SVGs to be like PDFs and render the same across devices. Is the issue that some renderers don’t implement the full spec, or that some implement parts incorrectly?

Yeah, I spent a bit of time trying to figure out some masking issues with a file I created in Inkscape but which chrome would butcher. Turned out to be opacity on a mask layer or something.