Let's say you are having some HTML elements that should only be enabled, if the browser sports certain features. By default, the element is disabled, like in the following example:

Open File...

Next, we check in Javascript is this certain feature is supported. If it is, we replace the disabled element with an active version.

if (isSupported()) {
    $('#item').replaceWith('Open File...');
}

Which results in this:

Open File...

Simple, but effective.