<link rel="icon">
The document's portrait on the tab, in bookmarks and in search results.
Without this line the browser goes guessing and requests /favicon.ico at the root. With it, the document chooses its own portrait: the sizes attribute lets the browser elect the right size, and an SVG covers every size at once.
The myth is the collection of dozens of PNGs that favicon generators push: today an SVG, one fallback PNG and an apple-touch-icon settle nearly everything. And there is the observatory's own trick: href="data:," declares an empty icon and silences the request — no 404 in the logbook.
<link rel="icon" href="/icone.svg" type="image/svg+xml">
<link rel="icon" href="/icone-32.png" sizes="32x32">