Looks like I understood the reason. It's not visible because it's inside the margin, and wiki does margins. :) Am I right?

Hello,

The following example does not work properly in wiki (the rule is missed), while in mkIV it's OK.

\starttext \startmarginrule The sound of a duck is a good demonstration of how different people listen to a sound. Everywhere in Europe the sound is equal. But in every country it is described differently: kwaak||kwaak (Netherlands), couin||couin (French), gick||gack (German), rap||rap (Danish) and mech||mech (Spanish). If you speak these words aloud you will notice that in spite of the consonants the sound is really very well described. And what about a cow, does it say boe, mboe or mmmmmm? \stopmarginrule \stoptext

http://wiki.contextgarden.net/Reference/en/startmarginrule

Regards, Vyatcheslav

-- Best Regards, Vyatcheslav Yatskovsky