[Webkit-unassigned] [Bug 156641] Safari incorrectly accepts invalid values for -webkit-background-clip (content, padding, & border); should require "-box" suffix

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=156641

--- Comment #4 from Karl Dubost <karlcow at apple.com> ---
When the bug was reported by Daniel (Hello Daniel :) ) in 2016 (6+ years ago) on both here and chromium 

Mike Lawther replied:
https://bugs.chromium.org/p/chromium/issues/detail?id=604023#c2

> chromestatus.com currently shows 15% of page loads contain -webkit-background-clip (https://www.chromestatus.com/metrics/css/timeline/popularity/178 - really high!). We don't have information about the usage of non'-box'-suffixed values, but without this data it's too risky to change behaviour.

Let's see what it looks like now:
https://chromestatus.com/metrics/css/timeline/popularity/178

It seems the usage has been decreasing a bit for `-webkit-background-clip` 

* background-clip 35.6766%
* -webkit-background-clip 7.5421%


Though the issue is really about the values content, padding, border, still being valid. And nobody has data about these.


* -webkit-background-clip: content (instead of content-box)
* -webkit-background-clip: padding (instead of padding-box)
* -webkit-background-clip: border (instead of border-box)


In https://chromestatus.com/metrics/css/timeline/popularity/178
the results might be a bit off too for the feature adoption on top sites (which is from 2018).

There is a list of sample URLs, but each time I checked the site CSS seems to have been updated to a modern syntax, even for things which essentially depends on WebKit behavior.

Example: 

```
.iv-drawer-content::-webkit-scrollbar-thumb {
    border: 4px solid transparent;
    border-radius: 8px;
    background-clip: content-box;
    background-color:rgba(102, 102, 102, .5)
}
```

Searching on GitHub, I mostly see correct form being used.
https://github.com/search?q=%22-webkit-background-clip%22+padding&type=Code


There was this recent bug on chromium too. 
https://bugs.chromium.org/p/chromium/issues/detail?id=1339290

> According to the compat spec, -webkit-background-clip is an alias for background-clip.
> 
> This issue involves supporting the 'text' value for background-clip and disallow the box-less values 'padding', 'border', 'content'.


https://wpt.fyi/results/compat/css-style-declaration-alias-enumeration.html?label=master&label=experimental&aligned&view=subtest&q=enumeration

There are a couple of things to fix related to background-clip behavior.
https://wpt.fyi/results/css?label=master&label=experimental&aligned&view=subtest&q=background-clip



1. Keep `-webkit-background-clip` as an alias of `background-clip`
2. Remove content, padding and border from the acceptable values could be a path forward. 

I wonder if there would be any regressions. 
Mozilla has site interventions to quickly intervene on eventual regressions. Safari has Quirks.cpp

There would be a number of tests to fix. 
https://searchfox.org/wubkat/search?q=-webkit-background-clip&path=LayoutTests&case=false&regexp=false

According to 
https://searchfox.org/wubkat/rev/340573c924b370891b7a0794ef39d4995d3670ee/Source/WebCore/css/parser/CSSPropertyParser.cpp#3224-3232

    // The values 'border', 'padding' and 'content' are deprecated and do not apply to the version of the property that has the -webkit- prefix removed.

so only the prefixed version has the old values. I wonder if there was any regression reported previously. It doesn't seem. Most of the issues seem to revolve around the text value.
https://bugs.webkit.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&f0=OP&f1=OP&f2=product&f3=component&f4=alias&f5=short_desc&f7=content&f8=CP&f9=CP&j1=OR&list_id=8503339&o2=substring&o3=substring&o4=substring&o5=substring&o6=substring&o7=matches&query_format=advanced&v2=webkit-background-clip&v3=webkit-background-clip&v4=webkit-background-clip&v5=webkit-background-clip&v6=webkit-background-clip&v7=%22webkit-background-clip%22

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