Why does Firefox perform a favicon lookup when the website doesn't request one?

Firefox Nightly
, ,
1

If I visit https://example.com/, I see:

GET
https://example.com/favicon.ico
[HTTP/3 404  150ms]


GET
	https://example.com/favicon.ico
Status
404
VersionHTTP/3
Transferred1.72 kB (1.26 kB size)
Referrer Policystrict-origin-when-cross-origin
DNS ResolutionSystem


    accept-ranges
    	bytes
    alt-svc
    	h3=":443"; ma=93600,h3-29=":443"; ma=93600,quic=":443"; ma=93600; v="43"
    cache-control
    	max-age=0, no-cache, no-store
    content-length
    	1256
    content-type
    	text/html
    date
    	Fri, 30 May 2025 22:10:15 GMT
    etag
    	"84238dfc8092e5d9c0dac8ef93371a07:1736799080.121134"
    expires
    	Fri, 30 May 2025 22:10:15 GMT
    last-modified
    	Mon, 13 Jan 2025 20:11:20 GMT
    pragma
    	no-cache
    quic-version
    	0x00000001
    server
    	AkamaiNetStorage

    Accept
    	image/avif,image/webp,image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5
    Accept-Encoding
    	gzip, deflate, br, zstd
    Accept-Language
    	en-GB,en;q=0.5
    Alt-Used
    	example.com
    Connection
    	keep-alive
    Host
    	example.com
    Priority
    	u=6
    Referer
    	https://example.com/
    Sec-Fetch-Dest
    	image
    Sec-Fetch-Mode
    	no-cors
    Sec-Fetch-Site
    	same-origin
    TE
    	trailers
    User-Agent
    	Mozilla/5.0 (X11; Linux x86_64; rv:138.0) Gecko/20100101 Firefox/138.0

Why is it requesting an .ico? I’ve been informed by IANA that none is set:

I think you will find it is typical user agent behavior for web browsers to request a favicon.ico from a web server in order to establish if there is an icon available for UI purposes. I suspect there is special logic when it comes to local files not to do this. Perhaps if you wanted to experiment you could put your example HTML in a remote server to see this behavior.

Either way, I don’t believe there is anything on our web server that is conditioning this behavior.

2

Most browsers are doing the same in order to follow the HTML specification. Here’s the revelant bit of information from the specification:

In the absence of a link with the icon keyword, for Document objects whose URL’s scheme is an HTTP(S) scheme, user agents may instead run these steps in parallel:

  1. Let request be a new request whose URL is the URL record obtained by resolving the URL “/favicon.ico” against the Document object’s URL, client is the Document object’s relevant settings object, destination is “image”, synchronous flag is set, credentials mode is “include”, and whose use-URL-credentials flag is set.

In other words, if a <link rel="icon" ...> tag is missing, the browser will attempt to get the file at /favicon.ico.

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