Add content to pseudo element

In pseudo elements as :before and / or :after is possible to use content property. Today we have a content property linked with flex. I'd like add a content to :before and :after.

What is the better design for that?

We could reuse the same content and verify the parameter. If the parameter is a String then we use it as a property to pseudo element, but if the parameter is a LengthOrNumberOrAutoOrNoneOrContent than we use it as property to flex.

Another option is to create a new property with another name as contentPseudoElement.

What is the best solution? Have any other?