Specificity

Hello, please, help a newbie.
I met opinion, that it is bad to use type-selectors and it is better to assign classes to elements in html (even if they are not needed) and use only class-selectors in css.

For example, instead of

header {
	...
}

assign in html
<header class="header">...</header>
and use class-selector like this

.header {
	...
}

But it says here, that “it’s best to style with a type selector, if possible”.
Is there any official recommendations?