[css-color-adjust-1] Should probably not honor non-url background images

[emilio]

We had to do this because too many people complained when we started doing the backplating.

• https://bugzilla.mozilla.org/show_bug.cgi?id=1617678
• https://bugzilla.mozilla.org/show_bug.cgi?id=1616440
• https://bugzilla.mozilla.org/show_bug.cgi?id=1617151

What we implemented is making background-image revert unless all the values are url() functions:

• https://searchfox.org/mozilla-central/rev/4ccefc3181f9d237ef4ca8bd17b4e7c101ddf7b5/servo/components/style/properties/cascade.rs#397

But open to other suggestions :-)

[tabatkins]

The other reasonable possibility @fantasai and I can come up with is to descend into gradients and adjust all their color-stops the same way as background-color (turning their color channels to match the forced bg color, but leaving their opacity intact).

(If anyone ever implements stripes() or image(), I suppose they'd need the same treatment.)

[smfr]

That's starting to sound a bit like color-filter (issue #2875).

[css-meeting-bot]

The CSS Working Group just discussed [css-color-adjust-1] Should probably not honor non-url background images, and agreed to the following:

• RESOLVED: Do not honor non-URL background images in forced color adjustments

The full IRC log of that discussion

<dael> Topic: [css-color-adjust-1] Should probably not honor non-url background images
<dael> github: https://github.com//issues/4916
<dael> emilio: When we impl we initially allowed all bg images like spec says. We had to revert and only allow url based bc/ things like gradients cause undesired effects. High contrast dark theme with white gradient you don't want to show
<dael> emilio: Not sure what Edge or new blink impl does, but we got a lot of complaints from high contrast users
<dael> AmeliaBR: End user pov gradients act more like background color than bg image which we'd preserve?
<dael> emilio: Right
<dael> Rossen_: Current impl is evolving in this respect
<dael> Rossen_: Issue you raised I checked one of the linked URLs. I didn't see why that didn't repro the issue.
<dael> Rossen_: Regardless two options you listed which is not honor or do something further and allow alpha blending of non-color bg to blend with override from system colors. That's an interesting approach but not sure effect on gradients and if that's more desirable
<dael> Rossen_: Either approach seems reasonable.
<dael> Rossen_: Before the call smfr linked to a prop from dino a while back
<dael> smfr: Linked b/c had a mech to desc how change colors and gradients. Instead of magic text you can say color-fliter-saturation:0
<dael> Rossen_: Defined BG color as a color-mix to be able to blend with computed color of system colors. That's the case and I was thinking if the color mix would be enough in this case
<dael> emilio: Need to color mix all colors in image. color-filter more tricky b/c if applies to system colors it's opposite. Need to prevent it from applying to UA colors which is not ideal
<dael> smfr: Had notion in webkit, but I don't think we specified anything on it
<dael> emilio: Doesn't that mean default color isn't filtered? I guess it depends
<dael> Rossen_: Could go more restrictive and not honor non-url bg images and than figure out if we can alpha-blend gradients
<dael> emilio: sgtm
<dael> astearns: Prop: Not honor non-url bg images for color adjustments
<dael> astearns: Or non-URL bg images people can use color adjustment MQ to make own changes?
<dael> fantasai: NO b/c we're not allowing them
<dael> astearns: But you can have rules in stylesheet o use differen gradients
<dael> AmeliaBR: Same as forced-color changing any other forced-color rule. It turns off forced color on certain element
<dael> fantasai: But that turns off all riles which is next issue
<fantasai> s/riles/rules/
<dael> astearns: Any objections to Not honor non-URL bg images in color-adjustments?
<dael> AmeliaBR: Can you say forced color?
<dael> Prop: Do not honor non-URL bg images in forced color adjustments
<dael> fantasai: I'm okay with it to start. Looking at other ideas in issue might be worthwhile, but we can start here
<dael> Rossen_: Agree. I think this is a reasonable first step on the path
<dael> RESOLVED: Do not honor non-URL background images in forced color adjustments

[atanassov]

Resolved by the WG and PR landed. Closing.