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Which line of HTML/CSS in the following webpage is invalid? Paste the HTML in http://validator.w3.org to see what it says.
<!DOCTYPE html>
<html lang="en">
        <title> 🐾 Pets R Us</title>
        <meta charset="utf-8">
            #header {
                background: url("dogs.jpg");
       <div id="header"><h1>Pets R Us</div>
       <a href="mailto:pets@rus.com">Email us today!</a>
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