We see a lot of requests and errors relating to favicon.ico not being found.

◀ RECV {"sessionId":1,"method":"Network.requestWillBeSent","params":{"frameId":"18","documentURL":"http://localhost:8907/favicon.ico","request":{"url":"http://localhost:8907/favicon.ico","method":"GET","headers":{"host":"localhost:8907","user-agent":"Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0","accept":"image/webp,*/*","accept-language":"en-US,en;q=0.5","accept-encoding":"gzip, deflate","connection":"keep-alive"},"hasPostData":false,"isLinkPreload":false},"requestId":"15350213115907","type":"Img"}}
<snip>
◀ RECV {"sessionId":1,"method":"Log.entryAdded","params":{"entry":{"source":"javascript","level":"error","text":"[Exception... \"Favicon at \"http://localhost:8907/favicon.ico\" failed to load: Not Found.\"  nsresult: \"0x80004005 (NS_ERROR_FAILURE)\"  location: \"JS frame :: resource:///modules/FaviconLoader.jsm :: onStopRequest :: line 227\"  data: no]","timestamp":1589032965858,"url":"","lineNumber":0,"stackTrace":{}}}}

Looking at LinkHandler, these requests can be controlled by some prefs.

In the Puppeteer case, some tests in network.spec.js are flaky because of the extra favicon requests.

I wonder what is more appropriate: modify the tests to consider favicon requests or set the associated prefs to false by default for automation clients like Puppeteer?

Some Puppeteer unit tests already filter out favicon-related requests, so there's prior art there. In Gutenberg, we'd have to also allow associated console messages about missing favicons.

I'll just update the tests as part of Bug 1599260