I noticed in line 8 he uses both an ID and a class on the same line, won't this conflict?

The id styles will overrule the class styles, because id's have higher specificity in CSS, but it isn't necessarily a bad idea to use both id's and classes together.

For example, you could have a class for all your buttons for certain default styles, and then an id for a specific button. Let's say I want all my buttons to have a certain padding and border radius, and they will default to be blue, but I'll overrule that for specific buttons:

.button {
  border-radius: 5px;
  background-color: lightblue;
  padding: 10px;
}

#cancel-button {
  background-color: red;
}

<button id="cancel-button" class="button">Cancel</button>

My cancel button will end up with the default properties from the button class (border radius, padding) but will have the red background color from it's id styles, not the default lightblue.