The 'mark' theme hook is static allowing only 'new' or 'updated' values/classes. This makes altering these values and classes impossible unless the theme function (soon to be template) is overridden. New core use case:.
Abstract the attributes of the open tag and value in between it and the closing tag to variables that are passed in the render array. We can keep the constants, but the attributes and value can be populated in preprocess based on this "status".