Why can identical on-screen widths paint differently? Compare transformed SVG paths in the live browser with a canvas control.
Same visible thickness, different local coordinate scales. Start here.
A second experiment from the original debugging work.
Four coordinate scales with the same target thickness.
Demonstrates why offscreen canvas measurements can miss a live SVG paint defect.
These are measurement and design prototypes. Results depend on browser version, zoom, device pixel ratio, and hardware. Record those conditions when comparing results.