Bug 284441 – WebKit ignores margin-top on 'legend' element (in fieldset)

• Component: Layout and Rendering
• Status: NEW

[Daniel Holbert]

STR:
1. Load https://bug1936475.bmoattachments.org/attachment.cgi?id=9442850

EXPECTED RESULTS:
100px of empty space above the legend.

ACTUAL RESULTS:
No empty space.

Firefox gives EXPECTED RESULTS; WebKit (and Chromium) give ACTUAL RESULTS.

It looks like WebKit and Chromium honor `margin` on all other sides of the legend; just not the top for some reason.

For reference, I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1936475 to investigate this on the Firefox side since we got a report (https://bugzilla.mozilla.org/show_bug.cgi?id=1935129) of a site that looks different-though-not-necessarily-broken in Firefox due to the site (maybe-inadvertently?) putting margin-top on a legend element. I also filed https://issues.chromium.org/issues/383368170 on this in Chromium.

[Karl Dubost]

This is even more interesting, because the margin is not added visually, but it is here in the getComputedStyle. 

```
window.getComputedStyle(document.querySelector('legend')).marginTop
# reports 100px
```

padding on the other end is taken into account on all sides. 

In `RenderBox.cpp` there is a special case for Legend, inside
`bool RenderBox::sizesLogicalWidthToFitContent(SizeType widthType) const {… }`
https://searchfox.org/wubkat/rev/a6aecb7cb3614074b11d5db8390698e4ef53b110/Source/WebCore/rendering/RenderBox.cpp#2956-2961

```
    // Button, input, select, textarea, and legend treat width value of 'auto' as 'intrinsic' unless it's in a
    // stretching column flexbox.
    // FIXME: Think about block-flow here.
    // https://bugs.webkit.org/show_bug.cgi?id=46473
    if (logicalWidth.isAuto() && !isStretchingColumnFlexItem() && element() 
         && (is<HTMLInputElement>(*element()) 
         || is<HTMLSelectElement>(*element()) 
         || is<HTMLButtonElement>(*element()) 
         || is<HTMLTextAreaElement>(*element()) 
         || is<HTMLLegendElement>(*element()))
        ) {
        return true;
       }
```

and in html.css

```
legend {
    padding-inline-start: 2px;
    padding-inline-end: 2px;
    border: none;
}
```

Ah probably more interesting `isLegend()` 
https://searchfox.org/wubkat/search?q=symbol:_ZNK7WebCore12RenderObject8isLegendEv&redirect=false
which triggers specific rules for layout computations including.
https://searchfox.org/wubkat/search?q=isExcludedAndPlacedInBorder&redirect=false

[Radar WebKit Bug Importer]

<rdar://problem/141267953>

[Karl Dubost]

Setting 

legend { margin: 100px;float:left} 

triggers interesting differences too. 
It's like a double margin was added inside the fieldset box.

[Karl Dubost]

Nice find Ahmad! Bug 9243
https://lists.w3.org/Archives/Public/www-style/2013Jun/thread.html#msg34