In CSS, you can select the first child of an element using the :first-child pseudo-class.

Here is an example:

css
ul li:first-child {
  /* styles applied to the first li element within a ul element */
}

In the example above, the ul li:first-child selector targets the first li element that is a child of a ul element. You can replace ul with any other parent element and li with any other child element to select the first child of that element.

Note that the :first-child pseudo-class selects only the first child of the parent element, regardless of the element type. If you want to select the first child of a specific element type, you can use the :first-of-type pseudo-class instead.

In CSS, you can use the ::before pseudo-element to insert content before an element. Here's an example:

css
p::before {
  content: "Note: ";
  font-weight: bold;
}

In the example above, the ::before pseudo-element is used to insert the text "Note: " before every p element on the page. The content property specifies the text that you want to insert, and the font-weight property sets the font weight of the inserted text to bold.

You can modify the content property to insert any text or HTML content that you want. For example, you could use the following code to insert a checkmark before a list item:

css
li::before {
  content: "\2713 "; /* Unicode code point for checkmark */
  color: green;
}

In this example, the content property uses the Unicode code point for a checkmark to insert the symbol before each list item. The color property sets the color of the checkmark to green.

Note that the ::before pseudo-element creates a virtual element that is inserted before the content of the selected element. The inserted content is not part of the actual HTML content, so it is not accessible to screen readers or search engines.

Yes, you can use the width property in a ::after pseudo-element in CSS. The ::after pseudo-element is a generated content element that is inserted after the content of an element. Here's an example:

css
p::after {
  content: "";
  display: block;
  height: 2px;
  width: 50%;
  background-color: black;
}

In the example above, the ::after pseudo-element is used to add a horizontal line after every p element on the page. The content property is set to an empty string, and the display property is set to block to create a new line. The height property is set to 2px to specify the height of the line, and the width property is set to 50% to specify that the line should take up half of the available space. The background-color property is set to black to specify the color of the line.

Note that the width property specifies the width of the ::after pseudo-element relative to its containing block. If the containing block has a fixed width, then you can use absolute units (such as pixels) to set the width of the ::after element. If the containing block has a relative width (such as a percentage), then you can use relative units (such as percentages) to set the width of the ::after element.