Allow SVG images in favicons

[Toflar]

Fixed restricting favicons to .ico only.
See https://caniuse.com/#feat=link-icon-svg

[leofeyer]

Thank you @Toflar.

[aschempp]

Are you sure this is correct? Browsers always request favicon.ico from the root site. Browsers might support a <meta tag with an SVG icon, but our root page icon is only for the fallback route, and that route cannot return an SVG icon imho?

[Toflar]

The way I understood the spec is that in case the link meta tag is missing, the browser shall request /favicon.ico and use that. Browsers require Content-Type: image/svg+xml to be present to handle the SVG accordingly which should happen automatically due to our mime type guesser. Hmmm, maybe that guesses image/svg though, so we might want to hardcode image/svg-xml...
We could also add a Vary: Content-Type although I don't see this super critical as our reverse proxy is going to be invalidate the cache entry anyway when you change the selection and the browsers likely cache whatever they want to anyway^^

[aschempp]

On caniuse.com, the Reference section links Specification to https://html.spec.whatwg.org/multipage/links.html#rel-icon, which is titled 4.6.6.8 Link type "icon". That section says

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.

2. Let response be the result of fetching request.

3. Use response's unsafe response as an icon as if it had been declared using the icon keyword.

So maaayyybeee that actually works …

[leofeyer]

and that route cannot return an SVG icon imho?

That's a valid point IMHO. Our favicon.ico route cannot return an SVG icon, can it? AFAIK, you need a link tag such as <link rel="icon" type="image/svg+xml" href="favicon.svg">.

@ausi /cc

[Toflar]

Of course it can? It does now?

[leofeyer]

But doesn't the browser expect image/x-icon if the file is called favicon.ico?

[Toflar]

No, that‘s the whole idea of this spec :)

[leofeyer]

I have tested this in Chrome, Firefox and Safari on Mac OS.

1. It does not work in any browser with the guessed MIME type image/svg.

2. Once I had overwritten the MIME type, Chrome and Firefox correctly loaded the SVG favicon. Safari did render the favicon when I requested /favicon.ico in the URL, however, it refused showing it in the tab bar.

3. Safari also refused to show the SVG favicon if if was added via <link rel="icon", so I guess that is ok.

So we definitely have to overwrite the MIME type with image/svg+xml to make this work.

[Toflar]

See #1554