The following example, when process with mkiv, will result in math
italic in the first two examples, where it should be roman and roman
bold.

\starttext
$\bfb Hello World$ % doesn't work

$\tfb Hello World$ % doesn't work too

$\tf Hello World$ % fine

$\bf Hello World$ % fine
\stoptext

-- 
 Khaled Hosny
 Arabic localiser and member of Arabeyes.org team
 Free font developer