It's a quite old property of CSS. You can use it to add some text into somewhere in the web page. Note that, it's only available within :before or :after block. For example, here is CSS:
.username:before {
content: "Username: ";
font-weight: normal;
font-style: italic;
}
.username {
font-weight: bold;
}
And HTML:
<span class="username">alice</span>
So now, the text "Username: " will display before the input even there is no that text within HTML source code:
https://jsfiddle.net/wyv0m4y4/0
How about the special characters? Sometimes you may want to add the unicode icon instead of pure text. Just use hexa value:
https://jsfiddle.net/wyv0m4y4/1/
Do you see it start with a "\" and then the numbers?
For more hex code: