作用:了解HTML结构
<!DOCTYPE html><html><head></head><body></body></html>
<!DOCTYPE html>
<html>
<head>
</head>
<body>
</body>
</html>
简单到在一些IDE中可以直接生成
<!DOCTYPE html>HTML标记<html>HTML标签<head>头标签</head><body>身体标签</body></html>
<!DOCTYPE html>HTML标记
<html>HTML标签
<head>头标签
<body>身体标签
有头标签有尾标签