Should a css class or an element selector be used in this case?

Say I have a green border color that is displayed multiple times on my page. Should I create a class with a border-color property set to green and then apply that class to any html element that needs the green border, OR, should I just select each html element using css selectors in one rule and give them all the green border color?

I basically want to know if there is a best practice to this. Should I create a class and include that a bunch of times in the markup, or should I just target all the needed elements in an external css file which would make the css file longer but keep all the "styling" in the css file?

Ex:

<header class="border">Header</header>
<footer class="border">Footer</footer>

.border {
  border-color: green;
}

OR

<header>Header</header>
<footer>Footer</footer>

header,
footer {
  border-color: green;
}