Five edge treatments over the four grounds that actually break a mark: pale, dark, mid-tone busy, and high-frequency detail. Same heart, same size — only the edge changes.
Grounds are CSS stand-ins for photography; what matters is the value range, not the picture. Real-photo test at the bottom.
Watch the pale and fine columns — that's where a mark either holds its silhouette or dissolves into the ground.
What I see: bare dies on pale and fine. The soft 30% adds haze without adding an edge — on the pale ground it's barely better than bare, which is the low-vision failure mode exactly. The tight dark shadow works but rounds off the notch. The keyline holds the silhouette on all four. The plate is the only one that's unconditionally safe — and it's the only row still showing a jadeite heart.
The real test. Small is where low-vision legibility is won or lost.
At 18px the soft shadow contributes nothing — it's below the size where a blur reads as an edge at all. The keyline still works, but 1.5px is now a real fraction of the mark and it starts eating the silhouette. Below roughly 20px, stop treating the mark and put it on a plate.
Two thresholds, both keyed to whether you control the ground.
Controlled ground → jadeite, untreated. Scrim the photo and the heart sits around 6.8:1, comfortably past the 3:1 that WCAG asks of a meaningful graphic. Fix the ground, not the mark.
Uncontrolled ground, 20px and up → white heart with a 1.5px keyline in --bbs-primary-ink, the darkest brand value. Crisp under magnification, holds the notch and the sparkle, no haze.
Uncontrolled ground, under 20px → plate. A solid --bbs-primary-ink disc, jadeite heart on top. Opaque is the point — at 84% over a pale photo jadeite drops to 2.2:1, under the bar; solid it is 8.6:1. The plate is the one surface in the system that is never glass. You own both sides of the contrast, so the ratio is guaranteed — and the thread comes back. This is the one I'd reach for most.
Note the pattern: the plate is the only option that keeps the heart jadeite and guarantees contrast. Every other route buys legibility by giving up the colour.
Drop the same image into both — a pale one is the honest test.