HTML input time, step attribute to set timestep by (say) x minutes?

Seems like Chrome 86 does not support this.

Yes. You can specify step attribute in seconds.

However, Opera, iOS Safari, and Google Chrome don't reject non-aligned user input. A user can specify 11:59 to such time field. Then, Opera and Google Chrome show a validation error message when he tries to submit the form.

It's possible. You just need to put seconds in step parameter, ex.

<input type="time" step="300"> <!-- 5 min step -->