I use ID's for something specific, classes are reusable. I find it easier to read and maintain code by sticking to this principle. I wouild also imagine in terms of performance using ID's will be faster as there is only one unique ID allowed per page whereas there can be multiple class selectors.

I can see from your profile you are heavily design orientated. When developing and manipulating the DOM with javascript you may want to apply for example a mouseover effect to all elements with the same class, but likely there will be times where you would only want to update a specific element following an action. The easiest traditional way to do this would be to reference that element in your data model by ID, then find it with javascript and update the contents. Something along the lines of...

var e = document.getElementById('element-to-update');

e.innerHTML = "Update element-to-update only";

vs

var e = document.getElementByClassName('element-to-update');

// Gets all elements on the DOM with the same class identifier

getElementById is more widely supported (IE 5.5 vs IE 9.0 is one example) than getElementByClassName so that in itself may be another reason to use an identifier rather than class.

Like Richard said. I use IDs when something needs to be specific and classes for more general use. You could technically use a class only one time and thus use it like an id. But watch were you call your class in your CSS. If you have multiple classes on an element and you want one to be only for that element, you have to make sure you always call that class farther down your css or it will get overridden by the other classes. Meanwhile if you had used an ID, it can take precedence over the class.

<div id="adiv">
  <ul>
    <li class="red"> I have my own class </li>
</ul>
</div>

div#adiv ul li {
/* takes precedence */
 color: blue;
}

.red {
color: red;
}

codepen of above Coyer over on CSS Tricks says it far better than I can: http://css-tricks.com/specifics-on-css-specificity/

Of course this is probably one of the articles you are referring to and he makes some pretty good points: http://oli.jp/2011/ids/

Hi everyone,

I responded to a similar question in the Treehouse Fans FB group. Here's what I said there:

Keep in mind that this is a beginner course, and while that's no excuse to instill bad habits, it's still important to teach a variety of concepts. After a student builds their first website, I would expect them to know about ID attributes and how to use them, so it's important to include a few examples. Lots of websites still use ID attributes and if a student gets a job, they'll likely be working with an existing code base. Best coding practices can come when most of the important concepts have been introduced. Hopefully that makes sense.