You can assign a custom attribute to your element, then select on that. So if you have an input where you want to store 200 characters but truncate at thirty, you could do input type="text" show="30" size="200"
in jquery, it might be $('input[show]').attr(show)) to select that value. The input[show] would only select elements with the show attribute. If you only want to select inputs with certain show values, you could do input[show="30"] or input[show="10"] if your show values were 30 or 10.
I shied away from that only because I wasn't sure about adding custom attributes and making my xhtml invalid, e.g. to a div block which doesn't have a show attribute in the spec.
Getting the argument itself is actually easy enough, I more ended up struggling to find an "appropriate" or natural interface for it within the html itself.
in jquery, it might be $('input[show]').attr(show)) to select that value. The input[show] would only select elements with the show attribute. If you only want to select inputs with certain show values, you could do input[show="30"] or input[show="10"] if your show values were 30 or 10.