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Please find below for the explanation:
Page headers: Site identity
Careful graphic design will give your Web site a unique visual identity. A "signature" graphic and page layout allows the reader to grasp immediately the purpose of the document and its relation to other pages. Graphics used within headers can also signal the relatedness of a series of Web pages. Unlike designers of print documents, designers of Web systems can never be sure what other pages the reader has seen before linking to the current page. Sun Microsystems's many Web pages and subsites all include a signature header graphic that includes basic navigation links:
www.sun.com
Even if you choose not to use graphics on your pages, the header area of every Web page should contain a prominent title at or near its top. Graphics placed above the title line should not be so large that they force the title and introductory text off the page on standard office-size display screens. In a related series of documents there may also be subtitles, section titles, or other text elements that convey the relation of the displayed document to others in the series. To be effective, these title elements must be standardized across all the pages in your site.
What are actually web headers? 
