alt-text
An element allowed inside either startflag or endflag to
provide alternate text for an image when the imageref attribute sets
an image to be used for flagging. When no alternate text is specified
for a revision flag, the default alternate text for revprop start
of change is a localized translation of "Start of change", and the
default alternate text for revprop end of change is a localized translation
of "End of change".