CSS :after not adding content to certain elements

Question

I'm having trouble understanding the behavior of the CSS :after property. According to the spec (here and here):

As their names indicate, the :before and :after pseudo-elements specify the location of content before and after an element's document tree content.

This doesn't seem to place restrictions on which elements can have a :after (or :before) property. However, it seems to only work with specific elements... <p> works, <img> doesn't, <input> doesn't, <table> does. I'm could test more, but the point is made. Note that this seems pretty consistent across browsers. What determines whether an object can accept a :before and :after property?

Answer 1

img and input are both replaced elements.

A replaced element is any element whose appearance and dimensions are defined by an external resource. Examples include images (<img> tags), plugins (<object> tags), and form elements (<button>, <textarea>, <input>, and <select> tags). All other elements types can be referred to as non-replaced elements.

:before and :after only work with non-replaced elements.

From the spec:

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.

With span:before, span:after, the DOM looks like this:

<span><before></before>Content of span<after></after></span> 

Evidently, that won't work with <img src="" />.