This time I was combining id's and classes. This is my HTML:

<div id="article" class="l-fixed">
	Article
</div>

CSS:

#article {
    width: 80%;
    float: left;
    border: 2px solid black;
}

.l-fixed#article {
    width: 600px;
}

Notice that there is no space in selectors: 

.l-fixed#article

Like this we can add class on the same element with id, otherwise, with space:

#article {
    width: 80%;
    float: left;
    border: 2px solid black;
}

.l-fixed #article {
    width: 600px;
}

I would need nested element, something like:

<div class="l-fixed">

	<div id="article">
		Article
	</div>

</div>