What is the difference between the id and class attributes?

• id must be unique in a document; it powers anchors (#section), label[for], aria-labelledby, getElementById.
• class is reusable and is the primary hook for styling.

<label for="email">Email</label>
<input id="email" class="field field--lg" />

On specificity: id (1,0,0) outranks class (0,1,0), so avoid styling by id - it becomes hard to override.