[css-values-4] Ratio of `0/0`?

[tabatkins]

So, the old <ratio>, since it only allowed integers, could meaningfully restrict its values to "greater than zero", so all ratios you could write were meaningful.

Now that we've relaxed it to <number>, we can't do that - can't do open ranges. So 0 is allowed.

And mostly that's fine! A ratio of 0/1 is just infinitely tall; a ratio of 1/0 is infinitely wide; CSS is okay with infinities. The "division results" are well-defined too - 0 and infinity, respectively.

But what about 0/0? We can't reject this at parse-time, since a calc() might be hiding a 0. But it definitely doesn't have a meaning; the division result is NaN.

Two possibilities I see:

1. Define that it produces an "invalid ratio", and places that take <ratio> have to define what that means. The 'aspect-ratio' property would define that it doesn't give an aspect ratio (same as "none"); the (aspect-ratio) MQs would define that it always fails to match.

2. Make it act the same as calc(0/0): it produces a NaN, which is censored into an infinity, so it's the same as saying 1/0 for the ratio.

I'm mildly leaning toward option 2. It's more consistent with 0/0 in general, and it means we don't need to ensure that every place that uses <ratio> defines what an "invalid ratio" does.

[tabatkins]

Never mind, I'm strongly in favor of option 2 actually; it ensures that aspect-ratio: calc(X/Y); and aspect-ratio: X/Y; have identical behavior in all cases.

[AmeliaBR]

Zero/infinite values were discussed #3757 (comment). My comment there mostly agrees with you above, although I seem to have skipped over the aspect-ratio: 0/0 issue. I'd personally prefer to treat that as invalid = no aspect ratio defined.

See also #4572, which came up when I was searching for the other discussion & is probably worth another look — when taking the aspect ratio from width/height attributes, do you ignore a zero value? Unlike the aspect-ratio property, here we have actual browser behavior to compare (though I doubt it would be a strong web compat issue to change, since it is kind of an extreme edge case).

[vwkd]

Might want to delete the reference in css-values-4#ratio since this issue is closed?

[fantasai]

Not sure Tab agreeing with himself but disagreeing with Amelia qualifies as an obvious bugfix. :) Also related is #5557 which was raised by @cbiesinger so flagging for WG discussion.