img srcset和大小无法正常工作

问题描述：

我目前正在尝试在我的网站上制作更好的响应式图像.经过一番研究，我发现了 srcset 和 sizes 属性.

I am currently trying to make better responsive images on my site. After some research I found the the srcset and sizes attributes.

我的目标是:

当屏幕尺寸大于600像素或小于300像素时，我要投放250x250的图片
当介于这两个值之间时，我要提供350x350的图片
我还想为像素比更高的屏幕提供更高分辨率的图像

这是我想出的.但这并没有像我所希望的那样起作用.始终会投放250x250的小型广告.

This is what I came up with. But it is not working as I though it would. The small 250x250 is always being served.

<img src="https://placehold.it/250"
  srcset="https://placehold.it/700 700w, https://placehold.it/500 500w, https://placehold.it/350 350w, https://placehold.it/250 250w"
   sizes="((max-width: 600px) and (min-width: 300px) and (min-resolution: 2)) 350px, (min-resolution: 2) 250px, ((max-width: 600px) and (min-width: 300px)) 350px, 250px" />

我还有一个问题:
在我的测试中，我发现调整窗口大小时浏览器将不会加载新图像，因此应提供其他图像.可以这样做吗?

And I have one additional question:
In my tests I found out that the browser won't load new images when the window is resized so that a different image should be served. Is it possible to do that?

答

好.经过一些艰苦的研究，我发现我不正确理解 sizes 属性.

Ok. After some tidious research I found out that I did not understand the sizes-attribute correctly.

以下是我如何实现自己想要的东西:

The following is how I can achieve what I want:

<img src="https://placehold.it/250"
  srcset="https://placehold.it/700 700w, https://placehold.it/500 500w, https://placehold.it/350 350w, https://placehold.it/250 250w"
   sizes="(max-width: 600px) calc(100vw - 50px), 250px" />

img srcset和大小无法正常工作

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