[malfist]

People make mistakes all the time. Breaking because somebody made a mistake that you can correct for just leads to unnecessarily fragile code.

What's the point of failing and breaking stuff if someone tells you their image is 144x144 but it's really 145x145? Who does that benefit?

[anyfoo]

The opposite is the case. Overall, being too lenient in what code accepts and applying heuristics will lead to way worse problems down the line. For example, you want your compiler to fail hard instead of saying: "Oh, this isn't a pointer, but I'm sure you meant well, I'm just going to treat it as a pointer!"

In this particular case, it seems to me that the hints serve no purpose and should be abolished, and in the meantime fully ignored, altogether. All necessary metadata is contained in the image file, and browsers should also be (relatively) strict in what image files with what metadata they accept, for security reasons alone.

And if they also went so far as limiting file size, the perpetrators that clog up bandwidth by putting up multi-MB favicons would catch on much earlier (or at all), too.

So what actually is the point of those hints, if browsers have to fallback anyway?

[notatoad]

The hints are not a hint in how to render the icon - browsers don't need hints for that. the hints are an instruction to browsers on which icon to download in the case where multiple icons are specified.

if you are safari and you don't know how to display SVG favicons, then you don't need to waste bytes downloading a favicon only to fail to display it. the HTML does not limit a site to only one favicon.

[anyfoo]

Why is that not done through the MIME type and using HEAD? The server is apparently much better able to figure out the MIME type through magic numbers and file extensions of the actual file, than the author (human or not) of the HTML, as we see.

The same headers also inform the browser that they can skip downloading a favicon that they consider too big, for example.

[scrollaway]

HEAD support is never a guarantee, and content type auto detection is just another kind of heuristics.

[anyfoo]

Ugh, HEAD is not being universally supported, at least for static content? Okay, I accept that this has value then.

As for the MIME type, for image types I'd say it's more than stable enough. Certainly much, much more stable than the 6.7% error rate mentioned in the article here, I'd be surprised if it was even 1%. If you double click on an image on your desktop for example, you can in almost all cases expect that it will be opened correctly. It ceases being a heuristic entirely if you tell the webserver that *.png is image/png, and only put PNGs with names ending in ".png".

Guess those are the reasons why I got out of web development in 10 years ago, everything's held together by scaffolding and needlessly wasteful and inefficient there.

[scrollaway]

You might be overthinking this. I agree with the philosophy that stricter is better, but in this case what do you expect broken hints to do?

They’re not used for rendering, they’re used for figuring out what to fetch. A HEAD request would be far less efficient than knowing ahead of time what to fetch: 1 request versus 2N+1 requests.

What you suggest sounds all fine but the entire web is user input for a browser, so no matter what, you need to define how to fail. If you can fail gracefully, you might as well do so, because a failure might not even be triggered by bad code/configuration on your side but simply by flaky network issues.