ID’s are unique
Each element can have only one ID
Each page can have only one element with that ID
When I was first learning this stuff, I heard over and over that you should only use ID’s once, but you can use classes over and over. It basically went in one ear and out the other because it sounded more like a good “rule of thumb” to me rather than something extremely important. If you are purely an HTML/CSS person, this attitude can persist because to you, they really don’t seem to do anything different.
Here is one: your code will not pass validation if you use the same ID on more than one element. Validation should be important to all of us, so that alone is a big one. We’ll go over more reasons for uniqueness as we go on.
Classes are not unique
You can use the same class on multiple elements.
You can use multiple classes on the same element.
Any styling information that needs to be applied to multiple objects on a page should be done with a class. Take for example a page with multiple “widgets”:
div class=“widget big”></div
No need to make a brand new class name here, just apply a new class right in the class attribute. These classes are space-delimited and most browsers support any number of them (actually, it’s more like thousands, but way more than you’ll ever need).