移动优先，默认你是用手机浏览该网页的，当你用pc浏览时，就会以min-width进行递增式媒体查询
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.wap {
font-size: 20px;
}
@media only screen and (min-width: 300px) {
.wap {
font-size: 40px;
}
}
@media only screen and (min-width: 700px) {
.wap {
font-size: 60px;
}
}
</style>
</head>
<body>
<p class="wap">移动优先</p>
</body>
</html>
pc优先与之相反，默认你是用pc浏览该网页，当屏幕缩小时以max-width递减式进行媒体查询
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.pc {
font-size: 60px;
}
@media only screen and (max-width: 700px) {
.pc {
font-size: 40px;
}
}
@media only screen and (max-width: 300px) {
.pc {
font-size: 20px;
}
}
</style>
</head>
<body>
<p class="pc">pc优先</p>
</body>
</html>