Conditional CSS

I got some great comments on my post about conditionally loading content.

Just to recap, I was looking for a way of detecting from JavaScript whether media queries have been executed in CSS without duplicating my breakpoints. That bit is important: I’m not looking for MatchMedia, which involves making media queries in JavaScript. Instead I’m looking for some otherwise-useless CSS property that I can use to pass information to JavaScript.

Tantek initially suggested using good ol’ voice-family, which he has used for hacks in the past. But, alas, that unsupported property isn’t readable from JavaScript.

Then Tantek suggested that, whatever property I end up using, I could apply it to an element that’s never rendered: meta or perhaps head. I like that idea.

A number of people suggested using font-family, citing Foresight.js as prior art. I tried combining that idea with Tantek’s suggestion of using an invisible element:

@media screen and (min-width: 45em) {
    head {
        font-family: widescreen;
    }
}

Then I can read that in JavaScript:

window.getComputedStyle(document.head,null).getPropertyValue('font-family')

It works! …except in Opera. Where every other browser returns whatever string has been provided in the font-family declaration, Opera returns the font that ends up actually getting used (Times New Roman by default).

I guess I could just wait a little while for Opera to copy whatever Webkit browsers do. (Ooh! Controversial!)

Back to the drawing board.

Stephanie suggested using z-index. I wouldn’t to do that in the body of my document for fear of screwing up any complex positioning I’ve got going on, but I could apply that idea to the head or a meta element:

@media screen and (min-width: 45em) {
    head {
        z-index: 2;
    }
}

Alas, that doesn’t seem to work in Webkit; I just get back a value of auto. Curses! It works fine if it’s applied to an element in the body but like I said, I’d rather not screw around with the z-indexing of page elements. Ironically, it works fine in Opera

A number of people suggested using generated content! “But how’s that supposed to work?” I thought. “I won’t be able to reference the generated DOM node from my JavaScript, will I?”

It turns out that I’m an idiot. That second argument in the getComputedStyle method, which I always just blindly set to null, is there precisely so that you can access pseudo-elements like generated content.

Dave McDermid, Aaron T. Grogg, Colin Olan, Elwin Schmitz, Emil, and Andy Rossi arrived at the solution roundabout the same time.

Here’s Andy’s write-up and code. His version uses transition events to fire the getComputedStyle check: probably overkill for what I want to do, but very smart thinking.

Here’s Emil’s code. I was initially worried about putting unnecessary generated content into the DOM but the display:none he includes should make sure that it’s never seen (or read by screenreaders).

I could just generate the content on the body element:

@media all and (min-width: 45em) {
    body:after {
        content: 'widescreen';
        display: none;
    }
}

It isn’t visible, but it is readable from JavaScript:

var size = window.getComputedStyle(document.body,':after').getPropertyValue('content');

And with that, I can choose whether or not to load some secondary content into the page depending on the value returned:

if (size == 'widescreen') {
    // Load some more content.
}

Nice!

As to whether it’s an improvement over what I’m currently doing (testing for whether columns are floated or not) …probably. It certainly seems to maintain a nice separation between style and behaviour while at the same time allowing a variable in CSS to be read in JavaScript.

Thanks to everyone who responded. Much appreciated.

Update: If you’re finding that some browsers are including the quotes in the returned :after value, try changing if (size == 'widescreen') to if (size.indexOf("widescreen") !=-1). Thanks to multiple people who pointed this out.

body::after { content: var(--bp, 'narrow');
} @media (min-width: 25em) { :root {--bp: '>=25em'}
}

@supports (--css: variables) {}