向 H1 元素添加两个类，使背景为粉红色和红色。
<!DOCTYPE html>
<html>
<head>
<style>
.intro {background:pink;}
.special {color:red;}
</style>
</head>
<body>
<h1 @(21)>My Home Page</h1>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<style>
.intro {background:pink;}
.special {color:red;}
</style>
</head>
<body>
<h1 class="intro special">My Home Page</h1>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<style>
.intro {background:pink;}
.special {color:red;}
</style>
</head>
<body>
<h1 class='intro special'>My Home Page</h1>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<style>
.intro {background:pink;}
.special {color:red;}
</style>
</head>
<body>
<h1 class="special intro">My Home Page</h1>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<style>
.intro {background:pink;}
.special {color:red;}
</style>
</head>
<body>
<h1 class='special intro'>My Home Page</h1>
</body>
</html>