Question

Anything that you can do with IDs you can do with classes.

So why is there a ID attribute then?

Yeah the ID is unique but a class can be unique too…

Answer 1

Firstly, from the HTML 4.01 Specification Section 7.5.2:

id = name

This attribute assigns a name to an element. This name must be unique in a document.

class = cdata-list

This attribute assigns a class name or set of class names to an element. Any number of elements may be assigned the same class name or names. Multiple class names must be separated by white space characters.

I understand that the spirit of your question is that by intentionally choosing unique class names, you can simulate the functionality given by id attributes, arguably making the id attribute redundant. Well, I can think of at least one thing that can be done only with ids; associating a <label> element with a form element using the for attribute. eg:

<label for="firstname">First Name:<label>
<input type="text" id="firstname" name="firstname" value="" />

This is a good practice for accessibility reasons. If you use <label for="id"> with checkbox or radio button elements, you get the added bonus of the labels being clickable. eg:

<label for="male">Male</label> <!-- the word "Male" is clickable -->
<input type="radio" id="male" name="sex" value="male" />
<label for="female">Female</label> <!-- the word "Female" is clickable -->
<input type="radio" id="female" name="sex" value="female" />