= attributed strings
attribute lengths are rounded to complete unicode codepoints
zero-length attributes are elided
consecutive attributes of the same type and value are merged
overlapping attributes of different types do not split each other
overlapping attributes of the same type but different values do split
empty string is allowed
empty string cannot have attributes
font family names are case-insensitive both in attributes and in descriptors
attributes are unique throughout a Unicode codepoint, not just to UTF-8 bytes
define what "it is an error" means in the case of uiFreeAttribute() and all uiAttributeValue() functions
does uiAttributeFamily() return a normalized string