[html] Why does overflow:hidden not work in a <td>?

That's just the way TD's are. I believe It may be because the TD element's 'display' property is inherently set to 'table-cell' rather than 'block'.

In your case, the alternative may be to wrap the contents of the TD in a DIV and apply width and overflow to the DIV.

<td style="border: solid green 1px; width:200px;">
    <div style="width:200px; overflow:hidden;">
        This_is_a_terrible_example_of_thinking_outside_the_box.
    </div>
</td>

There may be some padding or cellpadding issues to deal with, and you're better off removing the inline styles and using external css instead, but this should be a start.