Add a space (" ") after an element using :after

Turns out it needs to be specified via escaped unicode. This question is related and contains the answer.

The solution:

h2:after {
    content: "\00a0";
}

Explanation

It's worth noting that your code does insert a space

h2::after {
  content: " ";
}

However, it's immediately removed.

From Anonymous inline boxes,

White space content that would subsequently be collapsed away according to the 'white-space' property does not generate any anonymous inline boxes.

And from The 'white-space' processing model,

If a space (U+0020) at the end of a line has 'white-space' set to 'normal', 'nowrap', or 'pre-line', it is also removed.

Solution

So if you don't want the space to be removed, set white-space to pre or pre-wrap.

h2 {
  text-decoration: underline;
}
h2.space::after {
  content: " ";
  white-space: pre;
}

<h2>I don't have space:</h2>
<h2 class="space">I have space:</h2>

Do not use non-breaking spaces (U+00a0). They are supposed to prevent line breaks between words. They are not supposed to be used as non-collapsible space, that wouldn't be semantic.