[Webkit-unassigned] [Bug 201623] New: <bg-size> is allowed after a slash only if <bg-position> is specified in CSS background shorthand

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=201623

            Bug ID: 201623
           Summary: <bg-size> is allowed after a slash only if
                    <bg-position> is specified in CSS background shorthand
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: karl+webkit at la-grange.net

Example URL:
https://codepen.io/webcompat/pen/LYPLvow

Steps to reproduce the problem:
1. Go to https://codepen.io/webcompat/pen/LYPLvow
2. See how the dino image is displayed.

What is the expected behavior?
The image should not be displayed.

What went wrong?
Safari and Chrome are too lenient with the syntax.

Firefox gets it right. 


OS Version: OS X 10.14


This creates a webcompat issue for Firefox. 
https://webcompat.com/issues/38850
on this site at least.
https://www.t-online.de/nachrichten/ausland/id_86344692/vor-dem-brexit-johnson-will-offenbar-zwangsurlaub-parlament-gegner-sind-sauer.html


The CSS spec says:
`cover` attribute is [allowed only](https://drafts.csswg.org/css-backgrounds-3/#background) if a position has been specified before hand.


```
<bg-layer> =    <bg-image> 
                     || <bg-position> [ / <bg-size> ]? 
                     || <repeat-style> 
                     || <attachment> 
                     || <box> 
                     || <box>
```

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