The function s() contains the code

    if ($var == '0') {  // for integer 0, boolean false, string '0'

        return '0';

    }

and it is all your fault Petr : http://cvs.moodle.org/moodle/lib/weblib.php?view=diff&r1=1.482&r2=1.483

That screws up in the situation where you do

$value = format_float($mark, 2); // e.g. $value = 0.00

echo '<input type="text" value="' . s($value) . '" />';

Actual result: the input contains 0
Expected result: the input contains 0.00.

(Of course, my real code is more complex than that. The call to s() is in html_writer.)

I guess we need to change or remove that if in s().