Design for sixteen pixels, not for the preview
The favicon size that matters is 16×16. That is what sits in a browser tab, next to a bookmark, in a history list — the places people actually see it. Sixteen pixels is smaller than a typical letter of body text, and almost nothing survives at that scale except a single bold shape in high contrast.
The common failure is designing at 512 pixels, admiring it, and shipping something that becomes a grey smudge in the tab. Test at the real size. A monogram, a simple geometric mark, or one strong silhouette will read; a detailed logo with a wordmark will not.
High contrast against both light and dark browser chrome matters too. A dark mark on a transparent background disappears in a dark-themed browser. A solid background colour behind the mark solves it.
What the set contains, and why each part exists
Ten PNG sizes from 16 to 512 pixels cover browsers, bookmarks, Apple touch icons and Android home screens. Modern browsers pick the closest and scale, but supplying the exact sizes avoids them scaling badly.
The maskable icon is the one people forget. Android crops home-screen icons into its own shape — a circle, a squircle, whatever the device uses — and an icon designed edge to edge loses its outer ring. The maskable version pads the artwork inside a safe area so nothing important is cut.
The manifest tells a browser what to use when someone installs your site as an app: which icons, what name, what colours. The HTML file is the handful of link tags that connect it all, ready to paste.
No .ico file, on purpose
The classic favicon.ico exists for versions of Internet Explorer that no longer ship. Every browser in current use accepts PNG icons, which are smaller, simpler and support transparency properly. Including an .ico would add a container format nobody needs.
If you specifically require one — some old build tooling still expects it at the site root — any converter will wrap the 32 pixel PNG. It is not required for the icons to work.
Everything here happens in this browser tab. The image is decoded, redrawn and re-encoded by the Canvas API on your own device, and the result is assembled in memory and handed to you as a download. Nothing is uploaded, which you can confirm in about fifteen seconds with the network panel open.