:before pseudo-class does't works with images

I have following HTML:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>:before pseudo-class and images</title>
    <style type="text/css" media="screen">
        img {
            display: block;
            width: 640pt;
        }
        img:before {
            content: "Test";
        }
    </style>
</head>
<body>
    <img src="http://andreygromov.name/picture/flower/flower4.jpg" alt="Ваза с цветами" />
</body>
</html>

:before will not appear for image, but it does for any div.

Why ?

Update: I found this explanation in W3C:

Note. This specification does not fully define the interaction of :before and :after with replaced elements (such as IMG in HTML). This will be defined in more detail in a future specification.

Update 2:

I forget to mention - i need to visualize "alt" attribute of image with CSS.

img:before {
    display: block;
    content: attr(alt);
    background-color: #333;
    /* etc. */
}

Considering the specification you quoted, I guess this is a case where you have to wrap the image in an element and apply :before on that.

EDIT:

Considering, that the alt attribute is for alternative text and basically should confer the same information as the image does, wouldn't that mean you are duplicating the information for the user?

How about putting the information you want to display in the title attribute of the surrounding element and displaying that?

Example:

<style type="text/css" media="screen">
  .image:before {
    content: attr(title);  
  } 
  .image img {
     display: block;
     width: 640pt;
   }
</style>
<div class="image" title="Information here"><img ... ></div>

来源：https://stackoverflow.com/questions/5457825/before-pseudo-class-doest-works-with-images

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html

css

pseudo-class