Define how margin-top on fieldset's legend works

[zcorpan]

See demos

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/6107
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/6108

WebKit/Chromium ignore the margin

EdgeHTML honors the margin. The fieldset's border is centered within the border edge of the legend.

Gecko honors the margin. The fieldset's border is centered within the margin edge of the legend.

[zcorpan]

cc @MatsPalmgren @dholbert @bzbarsky @mstensho @thejohnjansen

[zcorpan]

Pages in httparchive with non-zero margin on legend

https://gist.github.com/zcorpan/85e8a646fdeb15f0580e37a92de3709f

[zcorpan]

http://www.umassonline.net/ is a page that renders differently between browsers. Firefox has more space above the fieldset/legend ("Step 1: Your Contact Details") compared to Safari and Chrome. Edge has as much space as Firefox, but the fieldset background is shown above the legend.

[zcorpan]

I've looked at the first 60. Although I filtered the list for pages that contain a fieldset, most of them don't contain a legend. The above was the only one I found with an actual clear difference.

This suggests to me that, web compat-wise, ignoring margin in the block-flow direction should be OK.

[zcorpan]

@MatsPalmgren are you ok with ignoring block margins?

[zcorpan]

@MatsPalmgren ping

[MatsPalmgren]

I tend to think that block-axis margins are useful also on a rendered legend. Ignoring them introduces an exception and exceptions are bad in general.

@tabatkins @mstensho WDYT?

[mstensho]

Interop for margins is horrible at the moment, so we're free to spec whatever. :)
Ignoring them seemed like one compelling option, making it easy to deal with.

But I agree that margins could be useful. But then let's specify that they don't collapse with anything on the outside (It's already implicit that they don't collapse with anything on the inside the legend, since legends establish a BFC).

We also need to spec how this affects alignment with the fieldset border. Are we still going to use the legend border box for alignment? I guess so? And the margins contribute to the size of the size of the block-start border area of the fieldset (just like the remaining parts (border box) of the legend), right?

[zcorpan]

OK I changed the spec in 16d86e5

[zcorpan]

Are we still going to use the legend border box for alignment? I guess so?

Yes. This is like Edge. Doing it like Firefox appears buggy since the legend ends up "floating" somewhere if you have different margins on each side.

[MatsPalmgren]

Sounds reasonable to me. (I filed bug 1490747 to fix the alignment.)