A macron is a type of accent; it consists of a horizontal bar over a vowel. It is used in several minority languages, of which NewZealand, Maori (mi) matters here. Maori, should really be written as Māori. However, the Latin character set that is predominantly used (see iso-8859-1(7)) does not have macron accents in it. This means that while people are using applications that don't use utf-8(7) -- predominantly older operating systems and applications such as Microsoft Office with the language set to English -- we will have trouble with the Māori language which uses all ascii characters except for the macron.
Also, things like PostScript and PDF have built in character sets that require extra fiddling with fonts and encodings if you use characters outside the default, which is what causes the problem described in HowToCreatePDFsFromWordWithMacrons.
Macrons are expressed in HTML via utf-8 escapes: