[css-align] justify/align have double effect in fieldsets?

[davidsgrogan]

HTML specifies that fieldset elements generate a principal box that establishes a box formatting context. Then, the anonymous fieldset content box inherits the justify/align properties from the fieldset element.

So, the justify/align properties take effect in the anonymous content box AND the principal box.

Things get hairy with column flexboxes:

<fieldset style="display: flex; flex-flow: column; align-content: end; width: 200px; min-height: 200px;">
  <div style="width: 100px; height: 100px;"></div>
</fieldset>

If align-content: end applies in both principal and anonymous boxes, then the 100x100 square will be on the bottom right of the fieldset: the principal box moves the anonymous box to the bottom because that's what align-content: end does in block flow, then the anonymous box moves the flex item to the right, because that's what align-content: end does in column flexboxes.

For simplicity, can we change css-align to require that {justify/align}-{items/content} properties don't take effect in fieldset-generated principal boxes?

This is a bit intertwined with the proposal to change how the anonymous content box's height is derived at whatwg/html#9123.