Worked examples beat rule lists in this subject

Whether a headshot works is hard to specify and obvious to see. That is why examples do more than checklists.

What examples reveal that rules do not

Rules say "use good lighting". An example shows you that side light which looked considered at full size has turned half a face into a grey mass at profile size.

Rules say "choose an appropriate background". An example shows dark hair dissolving into a dark wall, and the same person looking sharp against a mid tone.

The pattern in the ones that work

Unexciting: a tight crop, frontal light, and a background that separates from the subject's hair colour. Everything else is convention, and conventions differ by field enough that copying the wrong one is worse than copying none.

The comparison worth making

Put a portrait you like beside your own at the size both will actually be displayed. Most of the difference disappears, and what remains is the thing worth fixing.