Read the seam.

UVsDebug turns indexed or non-indexed triangular geometry into a labeled canvas. The helper is deliberately a browser-side diagnostic: it inspects UV attributes, not textures or renderer state.

UVsDebug labels each triangle and its vertex indices. It requires a triangular BufferGeometry with a uv attribute; missing UVs are a caller error rather than a silent fallback. Exact page composition, pixels, broad capability matrices, and whole-example fidelity remain separate.

Source: UVsDebug · official UV test