Move `css_class_len` to `GTW.AFS.MOM.Spec`
Change `ungettext` to use `ugettext` for `n == 1`

`gettext.ungettext` returns the untranslated string if the message
catalog doesn't contain a specific plural entry.
Disallow double quotes in attribute `description`.

The `description` property of attributes is used for the `title` of HTML
elements. Double quotes in `description` break the generated HTML.
Improve handling of `desc` in `do_element`
Fix test for computed Date.now `default_value`