The passport photo rules change twice in a child's life: at their first birthday and at their sixth. Under 6, they need not look at the camera or hold a plain expression. From 6, the adult standard applies in full.
Babies under one do not need their eyes open, and GOV.UK officially recommends laying them on a plain light-coloured sheet and shooting from above. Every relaxation, every rule that still applies, and how to actually get the shot.
Online applications take a digital photo or a photo code; paper applications need two identical prints. Which route each UK application accepts, what each one costs you in time, and the cases where you genuinely need both.
The ten faults that account for most UK passport photo rejections, what the published standard actually says about each one, and the specific thing to change before you submit again.
DVLA does not publish a photo standard of its own — it asks for a passport-type photo and points at HM Passport Office. Here is what that means in practice, and the places a licence photo differs from a passport one.
Every rule HM Passport Office applies to a passport photo, in one place: the 45mm by 35mm frame, the 29-34mm head height, the background, the expression, and the digital file specification for online applications.