The <header> element is used to organize all the content that goes at the top of a page. Within the <header> element, you might well include the <nav> content as shown in the next screenshot.
And, you will probably include standard (traditional) HTML tags such as headings and links.
So, the <nav> content—the links at the top of a page—are included in the <header> element when they are placed at the top of a page. Navigation elements placed in sidebars are also enclosed with the <nav> element, but since—in this case—they are not part of the header, they are not enclosed in the <header> element.
Within a <header> element, CSS styles associated with tags (<h1>, <h2>, and so on), or with the class <div> tags can be used to supply additional formatting rules for how content will be displayed.
With the aim of making content within a header more easily categorized, HTML5 includes the <hgroup> element...