The idea of classes (“class=”””) and identifiers (“id=””) is part of DOM manipulation. You can put in classes and id’s in each of the tags as you desire. Normally, the rule is an independent id for each tag, and a class for each GROUP of identical tags.
To make use of classes, you are required to use CSS, be it at the top of the page, or linked to another page. If you have your CSS at the top of the page, you do not need another CSS page, however most designers will frown from that.
See here for a more in-depth explanation: https://css-tricks.com/the-difference-between-id-and-class/
solved A confusion i have about HTML “classes” [closed]