Why do the :before and :after pseudo-elements require a 'content' property?

Question

Given the following scenario, why does the :after selector require a content property to function?

Notice how you do not see the pseudo element until you specify the content property:

.test {
    width: 20px;
    height: 20px;
    background: blue;
    position:relative;
}
			
.test:after {
    width: 20px;
    height: 20px;
    background: red;
    display: block;
    position: absolute;
    top: 0px;
    left: 20px;
    content:"hi";
}

<div class="test"></div>

Why is this the intended functionality? You would think that the display block would force the element to show up. Oddly enough, you can actually see the styles inside web debuggers; however, they do not display on the page.

Answer 1

Here are some references to various W3C specifications and drafts:

Selectors Level 3

The :before and :after pseudo-elements can be used to insert generated content before or after an element's content.

The :before and :after pseudo-elements

Authors specify the style and location of generated content with the :before and :after pseudo-elements. As their names indicate, the :before and :after pseudo-elements specify the location of content before and after an element's document tree content. The content property, in conjunction with these pseudo-elements, specifies what is inserted.

The content attribute

Initial: none

This property is used with the :before and :after pseudo-elements to generate content in a document. Values have the following meanings:

none - The pseudo-element is not generated.

---

The styling applied to ::before and ::after pseudo-elements affects the display of the generated content. The content attribute is this generated content, and without it present, the default value of content: none is assumed, meaning there is nothing for the style to be applied to.

If you don't want to repeat content:''; multiple times, you can override this simply by globally styling all ::before and ::after pseudo-elements within your CSS (JSFiddle example):

::before, ::after {
    content:'';
}