A whole class of rudimentarily customizable web applications operates by putting magic function calls into HTML/CSS templates, which you may then, if you have the patience and knowledge, attempt to customize. For example, a magic #showButton function may insert a useful system button, so you can move the button around on the resulting page by moving the magic function around in the template. Part of the magic of the function may be that the button is only actually inserted if the user is logged in, or it's Tuesday. Usually the logic and the display components of the magic function are not separated, though, so if you want the context-sensitivity, you have to take the button exactly the way they made it. If you insert your own button instead of the magic function, it will always appear on the page, even when the user isn't logged in, or it's Wednesday.

First, in the HTML template, wrap the magic function with a couple extra divs for use in the CSS. If the magic function is #showButton, for example, wrap it like this:

<div class=Override>
#showButton
<div class=Mine><input type=button value="My Button"></div>
</div>

Next, in the CSS template, add the following lines:

.Override>* {display: none}
.Override>.Mine {display: block}
.Override>.Mine:first-child {display: none}

The > selector matches immediate children, and the :first-child pseudo-class matches when the thing is the first child of its parent, so these three rules effectively say "Hide everything in the Override div, except show the Mine div, except hide even the Mine div if it is the first thing in the Override div."

<div class=Override>
<input type=button value="Their Button">
<div class=Mine><input type=button value="My Button"></div>
</div>

The Mine div is not the first child of the Override div here, so our third style rule doesn't apply, thus Their Button should be hidden, and our button should be shown.

<div class=Override>
<div class=Mine><input type=button value="My Button"></div>
</div>

Now the Mine div is the first child of the Override div, so all our rules apply, and our button is hidden along with theirs.