Sentinel: Your Web-Performance Watchman

Your logo is still an image… and so is mine!

Written by on CSS Wizardry.

Table of Contents
  1. Eww, what?!
  2. People and machines
    1. Featured case study: NHS
    2. Semantics is for machines
    3. Images are for people
  3. But, why?!

Almost two years ago I wrote an article about image semantics which covered the appropriate markup for your site’s logo. In short, it stated that logos are content and should therefore be marked up as img elements, not h1s, as is a very common practice.

The h1 method is used for two main reasons:

  1. It allows you to sprite up your logo.
  2. The misconception that a logo is somehow a heading.

My article – which I still stand by – debunked the second, but the former is a point of interest…

I received an issue on the CSS Wizardry repo asking me to go into more detail about how I handle things here:

I was recently re-reading your logo is an image article and I noticed you’ve taken a different approach on your new site. I was wondering if you could walk us through this.

tl;dr To balance the correct semantics with better performance.

So, I previously wrote that you should not use an h1 for your logo, but you should use an img. Here’s the markup that I use on CSS Wizardry right now (January, 2013):

<a href="/" class="site-logo">
    <img src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt="CSS Wizardry" class="s  s--csswizardry-logo">

You should notice that my src attribute doesn’t point to logo.png or similar, but to a Base64 encoded 1×1px transparent gif.

Eww, what?!

What I am doing here is pointing my img at a small transparent gif, and then using CSS to apply an SVG sprite as a background image. Here I am using an img which is semantically correct, but I am also being a little crafty and using CSS to apply the image that people actually see. Here is the (S)CSS:

    background-position:-5px -5px;
    @include vendor(background-size, 250px 250px);

    @include media-query(desk){
        @include vendor(background-size, 500px 500px);
        background-position:-10px -10px;

All I am doing here is defining a spriting object (.s) and then extending that class with .s--csswizardry-logo (with some responsive bits added in).

It is important to remember here that – even though it is only a 1×1px transparent gif – my logo is still an image.

Before I can get to why I am doing this, we need establish some groundwork…

People and machines

Featured case study: NHS

How I helped the NHS rapidly build a brand new product.

Read case study…

It is a gross oversimplification, but an important distinction we need to make when it comes to HTML and CSS is the one between people – our users – and machines. We typically (for the vast majority of cases) have two things accessing our sites; machines, like screenreaders and search bots; and people, who can see and interact with our website.

With this in mind, we typically build our sites considering those two scenarios. That’s the whole reason we build websites accessibly and care about code, but we still want the site to look pretty, so we design it up as well. This is us catering to both machines and users.

N.B. A visually impaired user visiting a website via a screenreader counts as a machine as it is the screenreader which deals with the code and then passes it along, rather than the user processing the site directly. I realise how offensive this may seem, calling visually impaired users machines, but it’s the only way I can word the fact that in this scenario it is a machine accessing the website – albeit controlled by a person – but not a person directly.

Semantics is for machines

Knowing this, we can begin to understand that only machines care about img elements. From a user’s perspective it makes no difference whether we use an img, a Flash object, a background image on a div, anything! The img is only useful for machines (screenreaders, search bots etc). This is why we have semantics.

Machines also utilise alt text; alt text isn’t for users to access, it is for machines to access and then pass to the user if necessary.

So, semantically speaking, all a correct implementation of an image needs to be is an img element with some alt text, as this is what machines require; machines (where we as front-end developers are concerned) have no idea, nor do they even care, as to what the actual pixel content of an image is. If you have an img element with an empty gif as its src and ‘CSS Wizardry’ as its alt text then all the machine knows is that this is an image with that alternate text; it has no idea as to the content of the image file.

Images are for people

The actual pixel content of an image does matter to people, the code does not. With this in mind, it doesn’t matter how we apply the styling to the machine’s img element, because users don’t care (though we’d obviously use CSS).

This is the important distinction; imgs serve two purposes:

  1. To tell a machine there’s an image, and some alt text if applicable.
  2. To display a graphic to a user.

By splitting these requirements right down the middle, we now understand how we can serve a 1×1px transparent gif as a semantically valid image, and validly apply the visual image via CSS.

But, why?!

It’s taken me a while, but here we are, why am I doing this?

The short answer is performance.

Anyone following CSS Wizardry’s posts, or me on Twitter, over these last couple of years will know that I have been getting more and more heavily involved in much larger projects where architecture and performance are paramount, and I love it!

One of my favourite things about this side of web development is how it fuses really impressive, interesting performance ‘hacks’ with ‘ugly’ markup. Two years ago I strove for semantic perfection and ‘hand-crafted’ my code; now I see code as an awesome powertool for creating big, fast websites. Spriting up empty elements is all part of this fun!

Spriting images is performance basics, we all know why we need it, but it’s important to keep the correct semantics wherever possible, hence me spriting up an img element.

If, semantically, a logo has to be an image, and you want to sprite this image up, then the best way to achieve this is to have the img itself being see-through. By using a 1×1px transparent gif then you get this see-through behaviour attached to a tiny image.

Now, this image is initially an extra HTTP request, a performance no-no, but:

  • It’s only a few bytes in size so it’ll be over the wire in no time.
  • Any subsequent usage is free because the browser won’t re-request it.
  • You can cache it forever because a 1×1px transparent gif will never need updating.

You would be forgiven for thinking you could just point your src at a nonexistent image, which would result in a 404 and not having to transfer any image back over the wire. The problem with this however is that – with there being no asset downloaded – there is nothing for the browser to cache. This means that if you had five sprited imgs on your page you’d also have five 404ing HTTP requests, and nothing getting cached for reuse. The initial overhead of one more HTTP request for dot.gif can soon pay off because that little image can be cached indefinitely and reused again and again. This isn’t even to mention the fact that the only reason you’re doing this to be able to sprite images, which is saving you loads of requests anyway!

It is important to note that spriting images won’t always be necessary and/or possible. I’m not saying you should do it all the time, and I am saying you quite often can’t (fluid images, for example). It’s just another possible tool to put in your toolbox.

So there we have it; spriting an img allows us to use the correct semantics and also take even more advantage of sprites! A machine sees an img labelled ‘CSS Wizardry’ (perfect) and a user sees my logo (awesome)! Everyone is happy, everything is correct.

Edit: As many people have suggested, you could – if your stats allow it – use a Base64 data-URI instead of a gif and save the HTTP request. This obviously isn’t cachable in the same way as an image is, but it is gzippable!

Did this help? We can do way more!

Hi there, I’m Harry Roberts. I am an award-winning Consultant Web Performance Engineer, designer, developer, writer, and speaker from the UK. I write, Tweet, speak, and share code about measuring and improving site-speed. You should hire me.

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