Template:Flexbox-item-header
Flexbox-item-header opens a wrapper for elements within a flexbox.
It sets several CSS properties that are meaningless outside of an HTML element with display: flex.
It must be closed with either Template:Flexbox-footer, or a </div>.
Flexbox-item-header accepts two parameters. order is required and sets the position of this flexbox-item relative to others; it accepts any integer. style allows for arbitrary CSS.
Frankly, the code for this is a mess, but it is necessary until the underlying technology is a more widespread web standard.