Understanding IDs vs Classes in HTML and CSS

If you hang around people who write HTML and CSS, sooner or later you'll hear the debate about ids and classes. It sounds mysterious at first, like two competing gangs in web development. In reality, they're just labels. But understanding how they differ saves you from a lot of weird styling bugs later.

In HTML, both id and class let you label elements so CSS (or JavaScript) can target them. The confusion usually begins with the simple question: "Why do we need two ways to label things?" The short answer is that ids are for unique things, and classes are for repeatable things. But that's a bit abstract, so let's slow down.

The Basic Difference

Imagine you're organizing a room. There might be one main door — that's unique. You'd label it "main door." But chairs? There might be eight of them. You wouldn't label each chair individually; you'd just group them. That's a class.

In HTML, id creates a unique label:

You're telling the browser "this element is special, there should only be one of these." Classes are more like grouping tags:

Now you can style all those button elements at once. It's efficient. And CSS likes efficiency.

Common Misconceptions

Where beginners often trip is treating id and class as stylistic choices instead of structural ones. They think id is for bigger, important elements and class is for smaller ones. That's not correct. The distinction is about uniqueness, not importance. A small element can have an id if it's one of a kind. A big component can have a class if it repeats.

CSS Specificity

Another subtle thing: CSS treats id and class differently in terms of selector specificity. Class selectors look like:

.button { color: blue; }

Id selectors look like:

#header { color: red; }

Ids override classes if both target the same element. This can either feel like a superpower or a headache depending on your perspective. Many developers learned specificity the hard way: they put a class style, it doesn't show, then they add an id style and suddenly things work. Later they try overriding that id style with another class and nothing happens. At that point they start adding !important tags everywhere, and that becomes a whole different rabbit hole.

Uniqueness Matters

Another key point: ids are meant to be unique across the entire page. You're not supposed to have two elements like:

Browsers won't explode if you do this. But CSS and JavaScript may behave unpredictably. JavaScript's document.getElementById() returns only the first one it finds. Tools weren't designed for duplicate ids. Classes solve that repetition problem cleanly.

In contrast, you can sprinkle the same class across a hundred elements:

Now styling them as a group becomes easy. You could say:

.price { color: green; font-weight: bold; }

and all your prices gain that look. If prices later change design, you update just one rule. That's the cascading benefit CSS was built for.

IDs as Page Anchors

Another area where ids enter the picture is linking within a page. If you've ever clicked a "Jump to Section" link and the browser scrolled down to the correct spot, that used ids. For example:

and somewhere below:

<h2 id="faq">Frequently Asked Questions</h2>

That trick only works because ids serve as anchors. Classes don't do that. You can technically fake it, but the web convention favors ids for anchors because there should be exactly one target location.

JavaScript Differences

JavaScript also likes ids when grabbing specific elements. It's direct and clear. Calling:

document.getElementById("header")

leaves no ambiguity. With classes you get collections, not single items. For example:

document.getElementsByClassName("button")

returns a group. Sometimes that's helpful, sometimes not.

Frameworks and Modern Development

And then there's frameworks. If you get into React, Vue, Svelte, or Angular, ids become less common because components handle uniqueness differently. Classes still remain important for styling. Tailwind, Bootstrap, Material design systems — all of them rely heavily on classes. Classes are basically the currency of CSS frameworks.

Stacking Multiple Classes

One subtle difference that beginners miss: you can stack multiple classes on one element. For example:

This creates composable styling. You could define:

.btn { padding: 10px; }

.large { font-size: 20px; }

.primary { background: blue; color: white; }

and these combine beautifully. Ids don't stack like that. You only get one id. That makes ids less flexible for styling patterns.

When to Use IDs

So why not skip ids altogether? Well, sometimes you genuinely want uniqueness. Think about:

If JavaScript needs to highlight or hide a section, grabbing it by id makes sense. You don't have to worry about selecting multiple elements accidentally.

Overusing ids, however, leads to CSS specificity wars. If you style something with #header, overriding it later becomes harder. Classes are easier to override and scale better when projects grow.

Best Practices

A lot of developers follow an unwritten rule: use ids for page-level anchors or JavaScript targets, use classes for styling and repetition. It's not mandatory, but it keeps projects clean. HTML itself doesn't enforce this. The browser won't yell. But future-you will thank current-you for not abusing ids.

Naming Conventions

There's also the topic of naming. Beginners often name things based on how they look, like:

Then three days later they change the color to green and now the class lies. Better naming reflects purpose, not appearance. Naming something price, card, label, button, wrapper, or section outlasts design choices.

Real-World Usage

Another moment in every beginner's journey is when they inspect professional websites and realize they're full of classes, not ids. If you pop open your browser's inspector right now and check out a big site like YouTube, Twitter, Amazon, or an online editor, you'll notice classes everywhere. Sometimes dozens of them, especially in component-based systems. Ids appear, but sparingly.

Once you spend more time building layouts, you start to appreciate that classes scale horizontally (repetition across elements) while ids scale vertically (unique locations). And both serve different mental models.

Accessibility Considerations

There's also accessibility in the mix. Labels in forms sometimes use for attributes that match input ids. For example:

<label for="email">Email</label>

This helps screen readers associate the label with the input field. Classes don't fill that role.

Eventually you reach a point where the id vs class question stops feeling like a mystery. Ids are for one-of-a-kind things. Classes are for groups. CSS treats them differently. JavaScript reads them differently. And browser behavior makes more sense when you match the right tool to the right context.

The nicest part is that this lesson sticks forever. Once you unlock this distinction, working with CSS becomes less frustrating, and debugging layouts becomes less of a guessing game.