The length field on the ban page is an input field, so it does not have an accessible innerHTML property like the textareas do. The value property should be used. Internet Explorer will throw JavaScript errors, Chrome will ignore it, but neither updates the shown value.

document.getElementById('acp_unban').unbanlength.innerHTML = ban_length[option];

should be:

document.getElementById('acp_unban').unbanlength.value = ban_length[option];

Additionally, the emphasis that is supposed to be added to excluded bans is only detected in FireFox and not Chrome or IE. Chrome reports that the font-weight value is overridden by the user agent stylesheet. IE's trace shows that it's applied, they just don't render it.