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Using CSS to add line numbering

When you want to display a code listing with HTML, you use a `<pre>` tag in order to indicate that the text is preformatted, then inside one or multiple `<code>` tags to specify it is a code.

Showing line numbers appears may be quite useful, and the methods to achieve this vary widely: many use Javascript or tables...

Yet it is possible to achieve this in a simple way, using only CSS and HTML. The line numbers won't be selected when the user wants to copy the code.

HTML

The only thing to do in our HTML code is to use a `<code>` tag for each line of code. This will produce a perfectly valid HTML code:

<pre>
<code>line 1</code>
<code>line 2</code>
<code>line 3</code>
<code>line 4</code>
<code>line 5</code>
</pre>

CSS

The `:before` pseudo-element will allow us to show an element before each line, while the CSS counters will allow us to count the lines.

We will first define a counter that starts at one for each block, then which is incremented at each new line of code:

pre{
    counter-reset: line;
}
code{
    counter-increment: line;
}

Then we display the number at the beginning of each line:

code:before{
    content: counter(line);
}

With Webkit browsers, selecting the code to copy it gives the impression that the numbers are selected (although fortunately they are not copied). To avoid this, simply use the property `user-select`:

code:before{
    -webkit-user-select: none;
}

It only remains to improve their style as desired. That's it!