A favicon is a small site icon used in places such as browser tabs and bookmarks. It is not limited to one 16×16 ICO file: browsers support several formats and may choose among declared sizes.
Declare the icon in the head
<link rel="icon" href="/favicon.png" type="image/png" sizes="48x48" />This assumes you provide a real 48×48 PNG at that path. Declare only sizes the file actually contains. An ICO can hold multiple sizes, and an SVG can be useful in supported browsers. Mobile home-screen icons have additional conventions.
Check the result
Create a simple recognizable square design and inspect it at small sizes. Open the page, check the browser tab, then use the Network panel to verify the icon URL and image response. Browser favicon caching can outlast an ordinary reload; confirm the requested asset rather than repeatedly changing unrelated markup.
Search has separate requirements
Google Search has its own favicon guidelines and supported formats; browser SVG support does not establish search eligibility. Use a stable crawlable icon URL, make the homepage discoverable, and consult the current search guidelines for size recommendations. Meeting them does not guarantee display in results.
A transparent background is a design choice, not a requirement. Test light and dark surroundings before choosing it. See link and head for metadata placement.
Sources
Reference 1 Reference 2 provide the technical details and current compatibility information.