Latin letters with macron are currently not rendered correctly on this site. The cause is that this site is using a version of Open Sans hosted by Stackexchange server, which only contains Latin-1 characters.
As letters with macron are essential in some rōmaji system, replacing the font file with a version includes Latin Extended (or fuller) glyph set is proposed.
Such files seem available at Google Fonts.
Test field:
A Ā I Ī U Ū E Ē O Ō
a ā i ī u ū e ē o ō
A Ā I Ī U Ū E Ē O Ō
a ā i ī u ū e ē o ō
