Are fxl property values restricted lists? #2039

Another curiosity looking at the properties is that the value lists all begin with the statement:

EPUB Creators MAY use the following values with the xxx property:

This suggests you may use other values, too. Shouldn't the statements say "EPUB Creators MUST use the following values..." or "The xxx property accepts the following values:"?

(The original FXL document had: "The following values are defined for use with the xxx property:" which is also kind of ambiguous about whether the lists are restricted or not.)

If these properties allow any value, then epubcheck is being overzealous in reporting unknown values as errors.