A pair checker is valuable precisely when its promise stays narrow. It answers what ratio two declared opaque sRGB colors produce. It cannot discover which pixels users actually see, what semantic role they serve or whether every state and requirement has been reviewed.
Transparency and changing backgrounds need compositing
An rgba foreground does not have one final luminance until it is composited over a known background. Shadows, overlays, opacity on ancestors and blend modes can change the visible result. Gradients and photographs contain many local pairs rather than one. ContrastCheck rejects alpha and image inputs instead of pretending that one typed color represents this rendering pipeline.
Real components contain several colors and states
A button may have label-to-fill, border-to-page and focus-indicator-to-adjacent comparisons, then different hover, pressed, disabled and high-contrast states. A text field can add placeholder, error and selected-text colors. The tool does not crawl DOM or CSS, choose required visual parts or enumerate those states. Build a deliberate inventory and test each applicable pair in context.
Accessibility review extends beyond color contrast
Keyboard operation, focus order, names, roles, labels, headings, reflow, zoom, errors, alternatives and assistive-technology behavior are outside a pair calculation. Even contrast exceptions require understanding content and purpose. Keep the copied result as calculation evidence, not as a certificate. Independent review remains necessary for any important design, policy or conformance claim.