Bold" is a style - when you say "bold a word", people basically know that it means to add more, let's say "ink" around the letters until they stand out more amongst the rest of the letters.
That, unfortunately, means nothing to a blind person. And on mobile phones and other PDAs, text is already bold because screen resolution is very small. You can't bold a bold without screwing something up.
is a style - we know what "bold" is supposed to look like.