Namespacing fragment identifiers

Written by Harry Roberts on CSS Wizardry.

I just stumbled upon something amazing. HTML allows colons (:) and periods (.) in ID tokens.

At first I thought this was awesome because, well, how cool is that?! But then I realised that neither of these are any use in CSS:

#foo:bar{ /* Looks for an element with an ID of foo and a pseudo-selector(class/element) of bar */ }
#foo.bar{ /* Looks for an element with an ID of foo and a class of bar */ }

So, whilst these are perfectly valid in HTML, they’re useless in CSS they can be styled with CSS. Kinda sucks, huh? But! If we know they’re okay in HTML and totally pointless in CSS, can we use that to our advantage?

Answer: yes!

Fragment identifiers

This is just the fancy name for when you link to an element with an ID on it, e.g. <a href="#content">Skip to content</a>.

I regularly use fragment identifiers when writing long articles with certain sections in them. The best and most relevant example I have is my epic on web type on Smashing Magazine.

Here I have a list of links to sections in the article, but you should notice I prefix each ID (and therefore its corresponding link’s href) with #tt-. This is so that I know that my sections will (almost definitely) not be picked up by Smashing’s CSS. If I had a section called #face and, for whatever crazy reason, Vitaly had a huge photo of himself with an ID of #face there would have been conflicts.

This was a situation where I needed IDs for no styling whatsoever, but rather to be used as fragment identifiers. And, to circumvent any of these potential hiccups, I namespaced all my fragment identifiers with #tt-, standing for technical type.

Here enters my little discovery…

If we want an ID that won’t be styled with CSS because we only want to link to it then we can use a colon or a period in there to:

1. Ensure that CSS cannot touch the element even if it wanted to.
2. Namespace the fragment identifier to show that it is meant as a link hook and give it some more human-friendly semantics.

Demo

I’ve made a demo page of this to a) prove that it works and b) show you how it works in situ.

Run it through the validator, see, it works!

I’ve also tested it in IE7+, Chrome and Firefox Mac and Win.

Problems?

Not that I can think of right away, but bear in mind that these elements cannot be styled via that ID.

For the most part the elements will be styled on an element-level basis (e.g. table{}, pre{} and so on) so styling them explicitly should not be too important.

If you do find you need to style them individually you could:

• Use a class.
• Revert to the tried and tested non-colon-or-period syntax.

A standard

I’ve never ever seen this anywhere before but I am already thinking it will be incredibly useful to me and any others who write documentation, articles or anything else with sections.

I propose a (loose) standard whereby you namespace your fragment identifiers with relevant information, so if you’re linking to a table in a document give the table an ID of #table:sales-figures, a figure showing a technical drawing an ID of #figure:engine-section, a section of an article an ID of #section:intro and so on.

Addendum

Ben ‘Cowboy’ Alman points out that escaping the colon will allow you to style the ID via CSS, as will escaping the period:

#foo\:bar{ /* Works! */ }
#foo\.bar{ /* Works! */ } 

By Harry Roberts

Harry Roberts is an independent consultant web performance engineer. He helps companies of all shapes and sizes find and fix site speed issues.

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