How to disable text selection highlighting using CSS?

What is CSS?
CSS is the language for describing the presentation of Web pages, including colors, layout, and fonts. It allows one to adapt the presentation to different types of devices, such as large screens, small screens, or printers. CSS is independent of HTML and can be used with any XML-based markup language.

What is Browser?
A browser is an application program that provides a way to look at and interact with all the information on the World Wide Web. The word "browser" seems to have originated prior to the Web as a generic term for user interfaces that let you browse (navigate through and read) text files online.

What is Text Selection?
Click where you want to begin the selection, hold down the left mouse button, and then drag the pointer over the text that you want to select. A word. Double-click anywhere in the word. A line of text. Move the pointer to the left of the line until it changes to a right-pointing arrow, and then click.

Here is code what you need.

<style>
.noselect {
  -webkit-touch-callout: none; /* iOS Safari */
    -webkit-user-select: none; /* Safari */
     -khtml-user-select: none; /* Konqueror HTML */
       -moz-user-select: none; /* Firefox */
        -ms-user-select: none; /* Internet Explorer/Edge */
            user-select: none; /* Non-prefixed version, currently
                                  supported by Chrome and Opera */
}
<style>