There can be a fair amount of discussion provoked by the phrase "CSS only" or "Built entirely in HTML and CSS" when it comes to tech demos. The discussion generally revolves around the fact that what the phrase actually means 99% of the time is "Using CSS for the graphical bits but tracking the mouse using JS" or "Using CSS transforms for most of it but using JS to calculate the values" or any number of variations on that theme.

Now, I'm not saying that's a bad thing at all, I quite happily called my interpretation of Super Mario Kart CSS-only even though without JS, Yoshi wouldn't make it to the first bend. What these demos are doing, essentially, is using CSS as the rendering layer where previously, JS would have been used. By doing so, you even get some nice hardware acceleration thrown in. Not too bad.

Why fall back?

The reason we fall back to JS for a lot of things is because CSS is a declarative language (see my post on CSS testing to learn more). However you say things should be is how they are. In CSS, you can't set a variable to one value, do something with it then set it to a different value and do something else. For a start, you don't have variables. Even if you did, the style sheet is read top-to-bottom before anything is done with it. The variable would always have the final value assigned to it, even the first time you used it. I'm simplifying a bit but not much. If you want to do anything clever, you generally need to resort to JS, an interpreted language.

Really CSS only

If you want to make something properly interactive but entirely CSS, you have to find a way to modify the state of the DOM so that different sets of styles can be applied to the same sets of elements. We actually do this all the time when we change the colour of a link on :hover. The user interacts with the page, the state of the page is changed slightly, a different style applies. There's a lot you can do now that you have a way to modify the page based on user interaction. Hooray for pseudo-classes!

An extremely important addition to the set of pseudo-classes available in CSS3 is the :target class. This becomes set on an element when the element is the target of a clicked link. Think of in-page jump links. When you click on one and the URL changes to 'blah.html#the-thing-you-clicked', the element with id="the-thing-you-clicked" gets the :target pseudo-class. Now you can affect it and its children with new styles. Now it becomes interesting, you can click on something on one bit of the page and it cause something to happen on another bit of the page.

Multiple-nested states

By nesting several elements around the thing you're intending to modify, you can now create a set of states entirely controlled by CSS. For example, with this HTML:

<div id="red">
  <div id="blue">
    <div id="green">
      <div id="yellow">
          <p>Hello.</p>
      </div>
    </div>
  </div>
</div>
<ul>
  <li><a href="#red">Red</a></li>
  <li><a href="#blue">Blue</a></li>
  <li><a href="#green">Green</a></li>
  <li><a href="#yellow">Yellow</a></li>
  <li><a href="#default">Default</a></li>
</ul>

And this CSS:

p {
    color:black;
}
#red:target p {
    color:red;
}
#blue:target p {
    color:blue;
}
#green:target p {
    color:green;
}
#yellow:target p {
    color:yellow;
}

You can change the colour of the text by clicking on the link. Without any JS.

See this on JSFiddle.

Shiny demos

This still isn't perfect. There are still going to be many things that JS is best for, calculations being one, keyboard input being another. To try and find the best way to show this off, I tried to update CSS Mario Kart to be entirely CSS and I almost got there but wasn't 100% successful.

• CSS-only Mario Kart

I then started to build a zoetrope in CSS, found someone else had already done it and decided to modify it.

• CSS-only Zoetrope

Maybe soon, the phrase "CSS only" will really mean "CSS only. And a bit of HTML."

Bookmarklets are the handiest little things. In case you don't know (which I'm sure you do), they're small chunks of JS that you store in your browser's Bookmarks or Favourites section which you can launch while looking at any web page. I write bookmarklets for all kinds of different tasks – navigating quickly around the build monitor at work, filling in tricky forms that my browser autocomplete doesn't handle, etc.

Here are a few of bookmarklets I find extremely useful for web development. To add them to your browser, simply drag the link on the title into your bookmark bar, hold shift down and drag or right-click and 'Add to Favourites', depending on what browser you're in.

Add JS to the page by URL

This will allow you to add any script you like to the current page. This can be particularly useful if you want to add a certain library or plugin to a page to investigate it further.

javascript:(function(){document.body.appendChild(document.createElement('script')).src=prompt('Script to add');})();

Add the latest CDN jQuery to the page

A variation on the above bookmarklet, this simply adds the latest version of jQuery. If you want to be able to play with a page and are familiar with jQuery, this will ensure that it is loaded and attached.

javascript:(function(){document.body.appendChild(document.createElement('script')).src='http://code.jquery.com/jquery-latest.min.js'})();

Add CSS to the page by URL

Add any stylesheet to the current page with a particular URL. This is handy if you want to demo variations to clients, I find. particularly if you predefine the CSS URL.

javascript:(function(d){if(d.createStyleSheet)%20{d.createStyleSheet(%20u%20);}%20else%20{var%20css=d.createElement(%27style%27),u=prompt(%27CSS%20to%20add%27);css.setAttribute(%22type%22,%22text/css%22);css.appendChild(document.createTextNode(%22@import%20url(%22+u+%22)%22));d.getElementsByTagName(%22head%22)[0].appendChild(css);}}(document))

Submit this page to the webdev subreddit

This isn't so much a web dev helper, more a general helper. I use this (or a variation thereof) to submit posts to specific reddits with fields prefilled. This bookmarklet defaults to the webdev subreddit.

javascript:window.location=%22http://www.reddit.com/r/webdev/submit?url=%22+encodeURIComponent(window.location)+'&title='+encodeURIComponent(document.title);void(0);

Add a CSS rule to the page

I can't remember whether I wrote this one or if I found it somewhere. The 'x<style>' is something from the middle of jQuery, though. Anyhow.

This allows you to add a style block directly into the page. This is useful for small CSS injections where the browser's web inspector is unavailable or incapable. Even though it's a single prompt that pops up, there's no reason why you can't past an entire stylesheet in there. I sometimes find myself doing that when I'm previewing designs on a production server.

javascript:(function(){var div = document.createElement('div'); div.innerHTML ='x<style>' +prompt('Style to add')+ '</style>';document.body.appendChild(div.lastChild);})();