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
It must be closed with either Template:Flexbox-footer, or a
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.