WPT details: https://wpt.fyi/results/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements See the result of fieldset-flexbox.html and fieldset-grid.html.
The FIELDSET element will support 'inline-grid', 'grid', 'inline-flex' and 'flex' keywords for CSS 'display' property.
The FIELDSET element can contain arbitrary elements, and it should support modern layout algorithms.