What a favicon does
Browsers request a favicon automatically. If your site does not serve one, the browser logs a 404 and falls back to a generic placeholder. That request appears in every visitor's waterfall, and the missing icon signals an unfinished build to anyone auditing your technical health.
Converting an image to ICO
Legacy ICO containers hold several raster sizes (16×16, 32×32, 48×48) in one file. Modern practice adds a 180×180 PNG for Apple touch icons and a manifest.json that references SVG for high-density displays. Use a converter that outputs the full set rather than a single 16×16 file.
Implementation checklist
Place the ICO at the root as favicon.ico for backward compatibility. Link the PNG and SVG variants in the head with explicit sizes attributes. Validate with a live site audit before you consider the task complete.