Proposal: Let's stop serving SVG images to Opera 12.
We support it as a Grade A browser (Browser support matrix at mediawiki.org), which means that we should do our best to provide the best possible experience.
In my opinion, the best possible experience involves not giving it any SVGs when possible.
- SVG background-images with border-radius sometimes disappear. Examples: MediaWiki UI checkboxes, in BetaFeatures: https://gerrit.wikimedia.org/r/#/c/132927/ in OOUI: T75517
- SVG background-images with background-size are sometimes rendered incorrectly. Examples: Echo flyout https://gerrit.wikimedia.org/r/#/c/177269/ (currently not merged yet), CompactPersonalBar dropdown icon F31554
It seems that SVG background-images are apparently immediately rasterized internally by Opera 12, and display pixelized when the user zooms in. <img> elements with SVG or inline <svg> are handled better, but we never use these in MediaWiki.
Doing this would involve a one-line addition to the .background-image-svg LESS mixin along the lines of change https://gerrit.wikimedia.org/r/#/c/132927/ (2b8f46c6dfbbe9a66029820f2b524b3aaf8fcddd). Uses in CSS are not worth hunting down, since these are probably the simple cases where it works correctly anyway.
The PNG images will not be embedded, resulting in worse performance :( We could consider inventing a separate mixin and using the two as appropriate, but I doubt anyone would consider this worth it.