if you set the same class for btns you can easily do:
Markup:
<ul>
<li class="button1 clickable" id="bold-button" title="bold">B</li>
<li class="button2 clickable" id="italic-button" title="bold">I</li>
<li class="button3 clickable" id="underline-button" title="bold">U</li>
<li class="button4 clickable" id="strikethrough-button" title="bold">S</li>
</ul>
JS:
$('.clickable').click(function(){})
. , , , ( , , jQuery $). , ..
$('.clickable').click(function(){$(this).css('font-weight','bold')})