<fieldset> resizes wrong; appears to have unremovable `min-width: min-content`

Update (25 Sept 2017)

The Firefox bug described below is fixed as of Firefox 53 and the link to this answer has finally been removed from Bootstrap's documentation.

Also, my sincere apologies to the Mozilla contributors who had to block removing support for -moz-document partly due to this answer.

The fix

In WebKit and Firefox 53+, you just set min-width: 0; on the fieldset to override the default value of min-content.¹

Still, Firefox is a bit… odd when it comes to fieldsets. To make this work in earlier versions, you must change the display property of the fieldset to one of the following values:

• table-cell (recommended)
• table-column
• table-column-group
• table-footer-group
• table-header-group
• table-row
• table-row-group

Of these, I recommend table-cell. Both table-row and table-row-group prevent you from changing width, while table-column and table-column-group prevent you from changing height.

This will (somewhat reasonably) break rendering in IE. Since only Gecko needs this, you can justifiably use @-moz-document—one of Mozilla's proprietary CSS extensions—to hide it from other browsers:

@-moz-document url-prefix() {
    fieldset {
        display: table-cell;
    }
}

(Here's a jsFiddle demo.)

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That fixes things, but if you're anything like me your reaction was something like…

What.

There is a reason, but it's not pretty.

The default presentation of the fieldset element is absurd and essentially impossible to specify in CSS. Think about it: the fieldset's border disappears where it's overlapped by a legend element, but the background remains visible! There's no way to reproduce this with any other combination of elements.

To top it off, implementations are full of concessions to legacy behaviour. One such is that the minimum width of a fieldset is never less than the intrinsic width of its content. WebKit gives you a way to override this behaviour by specifying it in the default stylesheet, but Gecko² goes a step further and enforces it in the rendering engine.

However, internal table elements constitute a special frame type in Gecko. Dimensional constraints for elements with these display values set are calculated in a separate code path, entirely circumventing the enforced minimum width imposed on fieldsets.

Again—the bug for this has been fixed as of Firefox 53, so you do not need this hack if you are only targeting newer versions.

Is using @-moz-document safe?

For this one issue, yes. @-moz-document works as intended in all versions of Firefox up until 53, where this bug is fixed.

This is no accident. Due in part to this answer, the bug to limit @-moz-document to user/UA stylesheets was made dependent on the underlying fieldset bug being fixed first.

Beyond this, do not use @-moz-document to target Firefox in your CSS, other resources notwithstanding.³

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¹ Value may be prefixed. According to one reader, this has no effect in Android 4.1.2 Stock Browser and possibly other old versions; I have not had time to verify this.

² All links to the Gecko source in this answer refer to the 5065fdc12408 changeset, committed 29ᵗʰ July 2013; you may wish to compare notes with the most recent revision from Mozilla Central.

³ See e.g. SO #953491: Targeting only Firefox with CSS and CSS Tricks: CSS hacks targeting Firefox for widely referenced articles on high-profile sites.

I’ve struggled for many hours with this, and basically, the browser is applying computed styling that you need to override in your CSS. I forget the exact property that is being set on fieldset elements versus divs (perhaps min-width?).

My best advice would be to change your element to a div, copy the computed styles from your inspector, then change your element back to fieldset and compare the computed styles to find the culprit.

Hope that helps.

Update: Adding display: table-cell helps in non-Chrome browsers.

Safari on iOS issue with selected answer

I found the answer from Jordan Gray to be particularly helpful. However it didn't seem to solve this issue on Safari iOS for me.

The issue for me is simply that the fieldset cannot have an auto width if the element within has a max-width as a % width.

Fix for issue

Simply setting the fieldset to have a 100% width of it's container seems to get around this issue.

Example

fieldset {
    min-width: 0; 
    width: 100%; 
}

Please refer to the below for working examples - if you remove the % width off the fieldset or replace it with auto, it will not continue to function.

JSFiddle | Codepen