WebKit sizes every <legend> to its content, not just the one its fieldset renders in the border. RenderBox::sizesLogicalWidthToFitContent() groups HTMLLegendElement with input, select, button and textarea, so any legend with width:auto shrinks to fit. Per HTML only the rendered legend (the first in-flow <legend> child of a <fieldset>) is sized that way; any other legend is an ordinary block. Steps: 1. Put <legend>hello</legend> directly in the body. 2. Compare its computed width to an identically styled <div>. Expected: both fill the containing block. Actual: the legend shrinks to its content, and is 0px wide when empty. wpt.fyi for html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/legend.html: Chrome 56/56, Firefox 56/56, Safari 49/56.