if you don't have macruby and hotcocoa already:
rvm use macruby
gem i hotcocoa
ok then you can run:
hotcocoa webviewbasic
this will generate a blank app copy application.rb in your lib/application.rb
and create an index.html file like this and put in "resources" dir:
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<h1>yo!</h1>
<p> awesome webapp bundled in osx</p>
</body>
</html>
rake run
enjoy!