A way to pre-load images since many UIs depend on the size of an image.

Many UIs depend on the size of an image. For example, a div with flex=min contains an image. Another example is an upload load with a big image. Currently, the developer has to specify the width and height explicitly. However, sometimes it is better to be decided by the real size of the image. It means we have to introduce a feature that can pre-load images, such as

<?image src0="/img/img0.png" src1="/another/foo.png"?>

<button upload="true" image="/img/img0.png"/>