Optional time in <input type="datetime-local" required>

It is possible to set a default value for the time only or make the time optional for a datetime-local input (see this article as reference: MDN web-docs):

<input type="datetime-local" value="2017-06-01T08:30" required/>

How can I preload only the time or make the time, but not the date optional, e. g.:

<input type="datetime-local" value="YYYY-MM-DDT12:05" required/>

This should look something like this:

It seems there is no way to do it in a single datetime input, but you can set default value for time input, so use an empty date input and a time input with default value, then use some CSS to make them like they are one input.

#time{
  border-left:0px;
  margin-left: -6px;
}
#date{
  border-right:0px;
}
*:focus {
    outline: none;
}

<input type="date" required id="date">
<input type="time" required value="07:30:00" id="time">