[css-backgrounds-4] default background-origin for `border-area` in shorthand

[fantasai]

In #10907 (comment) @jsnkuhn wrote:

FYI, the other property reset i've used every time with border-area is background-origin: border-box;. Considering resetting border-box as an initial background-origin value might be of use as well since im my experience most devs are not familiar with that property.

https://codepen.io/jsnkuhn/pen/eYwoGgr?editors=1100
https://codepen.io/jsnkuhn/pen/LYKXGPo
https://codepen.io/jsnkuhn/pen/WNqaeOV
https://codepen.io/jsnkuhn/pen/OJeoapM

There's two relevant questions

• should background-origin check background-clip to compute a smart default value
• in the background shorthand, when one box keyword is specified it's applied to both background-clip and background-origin (spec); but border-area is not a valid keyword for background-origin, so should we default it to border-area.

I think we should definitely fix the latter. Unsure about the former, though it would certainly be helpful to authors.

[tabatkins]

(I presume the end of your second bullet point means to say border-box.)

Yes, I definitely think that background: url(...) border-area; should default -origin to border-box. We already magically match up the two properties when you specify only one value (and while nothing special happens for text, the initial padding-box value for origin is likely what you expect anyway), and a border-box origin is clearly the similarly-intended partner to a border-area clip. (This would also apply to a border-area text value, I presume.)

I'm mildly against doing this with the initial value (rather than just via the shorthand); we don't match up the values today for any of the values; that is, if I set background-clip: border-box;, I'll confusingly still get a background-origin: padding-box; by default, and have to remember to correct it. I don't think border-area is meaningfully different from border-box here, so we shouldn't add novel magic for it.

[css-meeting-bot]

The CSS Working Group just discussed [css-backgrounds-4] default background-origin for `border-area` in shorthand, and agreed to the following:

• RESOLVED: Using 'border-area' in the background shorthand (and omitting origin) defaults the origin to border-box

The full IRC log of that discussion

<TabAtkins> fantasai: in the bg shorthand if you specify a single keyword like 'border-box', it sets both clip and origin to that keyword
<TabAtkins> fantasai: but 'border-area' keyword is only valid for clip, not origin
<TabAtkins> fantasai: want to clarify that in that case, origin defaults to border-box
<TabAtkins> +1 to this from me
<kizu> +1, makes sense
<oriol> +1
<TabAtkins> Rossen0: objections?
<fantasai> background: ... border-area;
<emilio> A bit unfortunate that it's different from background: ...; background-clip: border-area
<emilio> But yeah
<TabAtkins> RESOLVED: Using 'border-area' in the background shorthand (and omitting origin) defaults the origin to border-box
<TabAtkins> fantasai: second, do we want some way to make things default if you specify the longhand?
<TabAtkins> fantasai: argument for, it's unfortuante/confusing to get it set to the "wrong" value
<TabAtkins> fantasai: argument against, it's not something we're currnetly doing
<TabAtkins> Rossen0: okay, will leave that part of the discussion
<fantasai> for other values of background-clip (though maybe we should have)