header{
    width: 1000px;
    background-image: url(img/油田井场.jpg);
    text-align: center;
    color: white;
    padding: 26px 0;
    margin: 0 auto;
}
nav{
	width: 1000px;
	margin: 0 auto;
	display: flex;
	background-color: #003056;
}
nav a{
	display: block;
	width: 250px;
	text-align: center;
	color: white;
	font-size: 20px;
	padding: 15px 0;
	text-decoration: none;
}
nav a: hover{
	background-color: #ef220f;
}

article{
	width: 1000px;
	margin: 30px auto;
	font-size: 18px;
	line-height: 1.6em;
}
h3{
	text-align: center;
}
h4{
	text-align: left;
}
<!p{
	text-indent: 2em;
}>
ol{
	list-style: decimal inside none;  
    margin-left: 0em;
}
ul{
	list-style: disc inside none;  
    margin-left: 0em;
}
dl{
	margin-left: 2em; 
}
.imgbox{
	text-align: center;
}
dt .imgbox{
	text-align: center;
}
footer{
	background-color: black;
	text-align: center;
	padding: 20px 0;
	color: #939393;
}

div a{
	text-decoration:none;
	color: #939393;
	display:inline-block;
	height:20px;
	line-height:20px;
}