Adding a space to a phone number with just CSS - html

Question

I have a page which generates a phone number in HTML, like this:
<div class="phone">01987123456</div>
What I want is to simply put a space inside the number, like so:
01987 123456
The generated number and HTML will always be the same, but I only have access to client side code (HTML / CSS / Javascript / etc).
I want to find a way of achieving all of this without using Javascript if possible, so Ideally I am looking for an answer in CSS or HTML.
I'm pretty sure this could be done fairly easily in Javascript, but the client wants to make sure the phone number is formatted correctly even if Javascript is disabled (don't ask).
I want the most effective and efficient way of changing the number to what I want. If someone can figure out how to add brackets to the number (like this: [phone]) as well as the space using just CSS/HTML you will immediately get marked as correct as well as my eternal gratitude.
EDIT:
I get that CSS is for design, Ive been a web developer for 15+ years. I could really do with a CSS hack to produce what I want, and explaining to the client the basics of web design is unfortunately not an option (they think they know better and I am in no position to dictate anything to them). I'm in a bit of a nightmare situation, and I need your help!
I know that content can be added to a page with CSS using content. I am aware of the ::first-letter method that #gillesc mentions in the comments. I was hoping something like this might help me.
The client uses modern browsers so a CSS3 solution would be fine.
And no, I cant change the outputted HTML.

Answer 1

I was interested to see if this could be done with CSS, even if it shouldn't be done! The following is quite hacky, ideally the phone number would be formatted server side or, if that isn't an option, with JavaScript.
A few caveats:
This requires an attribute to be added to .phone for the pseudo element to use. This may or may not be a deal breaker given that you seem to have limited access to the HTML
If the phone number is not in a suitable format (e.g. something like 01 987123456) it will not display correctly
A nasty little hack is used for IE as it doesn't calculate the width of the pseudo element correctly using ch for some reason. Credit to SW4 for this: https://stackoverflow.com/a/20541859
A solid background colour is required
The general idea behind this is as follows:
.phone
text-indent: 1ch; on .phone moves the whole text to the left by one character
.phone is set to position: relative; to allow the pseudo element to be positioned relatively to it
white-space: nowrap; ensures that this doesn't wrap onto a new line if there is a break in the number
.phone:before
background-color: white; masks the digits in .phone
border-right: 1ch solid white; hides the sixth digit in .phone, in effect this is the space
content: attr(data-phone); uses the data-phone attribute on .phone to populate the pseudo element with the same number
left: 0;, position: absolute; and top: 0; are used to position the pseudo element
overflow: hidden; hides any characters over the 5 character limit
text-indent: 0; resets text-indent: 1ch; set on .phone
width: 5ch; ensures that the pseudo element is only 5 characters long
The weird media query is the hack to target IE
Tested and working in FF 38.0.5, Chrome 43.0.2357.124 m and IE 11. Browsers not supporting the ch unit (such as Opera 12.17 and Windows Safari 5.1.7) seem to show the phone number in its natural state.
.phone {
position: relative;
text-indent: 1ch;
white-space: nowrap;
}
.phone:before {
background-color: white;
border-right: 1ch solid white;
content: attr(data-phone);
display: block;
left: 0;
overflow: hidden;
position: absolute;
text-indent: 0;
top: 0;
width: 5ch;
}
#media screen and (min-width:0\0) and (min-resolution: +72dpi) {
.phone:before {
width: 5.8ch;
}
}
<div class="phone" data-phone="01987123456">01987123456</div>
JS Fiddle: https://jsfiddle.net/scarjnb1/

Answer 2

It's not possible using CSS, just JavaScript. Then it'd be:
<div id="phone">01987123456</div>
<script>
var el = document.getElementById('phone');
phone.innerText = phone.innerText.replace(/^(\d{5})/, '($1) ');
</script>