Class selector or ID attribute

Don't exactly know what lesson you are working on put it's best practice to use an id if your element is unique such as having one slider on a website and giving it the id of "slider". You use a class when you have multiple elements that can use the same styling.

Hello! in CSS Basics / Basic Selectors - you will learn more about this. The id's have to be unique. Use class instead if you want to select multiple elements to apply an style for example.

Always a good idea to only use IDs when you know you'll need them and when there isn't a chance of duplicating the same element elsewhere on the same page. A lot of developers will simply default to using a class selector rather than an ID selector for this reason. You never know when it might be a good idea to reuse the styles you've already written!

Check out this page for a bit more information.

In general use the class selector if two or more elements, on a single page, will need to use the same rule. For example: <div class="sameStyles">Hi</div> <div class="sameStyles">Hi</div>

Use the id selector when there will be only one element, on the page with that id attribute value, that will need to use the rule. For example: <div id="first">Hello</div> --> The value of "first" for the id attribute can only be used once on the page. <div id="second">Bye</div> --> The value of "second" for the id attribute can only be used once on the page.

When you get into the DOM the getElementById() is more browser backwards compatible then getElementsByClassName().

You want to use class if your planning to reuse it.

ID is you know it's the only element your changing or selecting.