I volunteer with seniors helping them with online problems. The lack of clear guidance what is and what is not an active element in a web view, responsive to touch, is their biggest cognitive burden. They really struggle.
Seniors make up 4-5% of the online population. As a cohort they're growing. Cognitive decline, visual acuity, memory all fade. So these are otherwise competent people who made good livings, now struggling with design choices with consequences for them.
There is simply no single abiding rule. It may be a red box, or a yellow circle, it may be words or a logo or a skuomorphism in cartoon sense.
I would suspect toddlers are the same. Who knows?
Gov.uk is exemplary for understanding this problem.
Material design isn't orthogonal in this, because it's flat aspects de-emphasis any clear distinction where prior coding models tended to have a gui look to buttons which lay outside this design imperative.