Can I make custom elements in HTML? If so, how?

Question

I would like to make an element

for example something like

<header>
</header>

or like

<headerItem>
</headerItem>

and I would like to know how to style them in CSS

Answer 1

You can use <header> as you suggested and to reference it in css, instead of using "." or "#" to reference a class or ID respectively, you just use the element name, for example, for <header>

The css would look like this:

header {
   background-color: black;
   /* Whatever */
}