Bootstrap将屏幕分为4个等级:
1.超小屏幕 (宽度小于768 px), 显示宽度 100%;
2.小屏幕 (宽度在768px ~ 992px), 显示宽度 750px;
3.中等屏幕 (宽度在992px ~ 1200px), 显示宽度 970px;
4.大屏幕 (宽度大于1200px), 显示宽度 1170px.

js实现:

window.addEventListener("resize", function() {let clientW = window.innerWidth;let container = document.querySelector(".container");if( clientW >= 1200) {container.style.width = "1170px";} else if ( clientW >= 992 ) {container.style.width = "970px";} else if ( clientW >= 768 ) {container.style.width = "750px";} else {container.style.width = "100%";}
});

CSS3媒体查询实现:

@media screen and ( max-width: 768px ) {.container{width: 100%;}
}
@media screen and ( min-width: 768px ) and ( max-width: 992px ) {.container {width: 750px;}
}
@media screen and ( min-width: 992 px ) and ( max-width: 1200px ) {.container {width: 970px;}
}
@media screen and ( min-width: 1200px ) {.container {width: 1170px;}
}