Tuesday, May 15, 2012

Customizing InDesign's EPUB files without cracking them open!

I'm working on my presentation about Enhanced EPUB for PePcon on Tuesday afternoon and I was playing with the newly released InDesign CS 6 to make sure that everything works as expected. But it didn't. For some reason, when you place an audio file, iBooks displays the controls so small that the only thing you can do is choose Airplay, but not the actual Play button (if you're on an Airplay network). It's pretty frustrating and would completely ruin InDesign's much improved multimedia function if not for one thing.

You can fix it without cracking the EPUB file open. One of InDesign CS 6's other new features is that you can choose additional CSS files to apply to your exported EPUB. All you have to do is create a CSS file that fixes the code that InDesign supplies, and voilĂ , the audio controls reappear. You don't have to unzip, or worse, rezip.

The key lies in the letter "C" of "CSS". It stands for Cascade. The Cascade means that when there are multiple sets of style rules for a given bit of text, there is a defined hierarchy for which rules will win out. I actually see it as a sort of waterfall in my head with the more “important” rules flowing over and covering the lesser ones.

To make the cascade work, you have to understand the slightly complicated topic of CSS inheritance: given two competing rules, which one wins? I talk about this in some detail in my HTML book. The very abbreviated answer is that you have to take into account specificity (that is #id overrules .class overrules p) and location (that is, inline overrides local overrides earlier, overrides imported), as well as taking into account the !important tag.

In our example, InDesign creates this code for audio elements. Note that this is the default size if you don't change the poster image for the audio element.

audio.frame-5 {

But when I open that document in iBooks on the iPad, this is what I see:

Default audio controls

And when I click it, all I can select is which Airplay device I want. I can't actually play the audio.

Default audio controls-airplay

So, I open a new text document. InDesign 6 lets me add additional CSS documents and will automatically load them after the CSS that it creates itself. That means that since my styles are later, as long as the specificity is the same, my styles will override InDesign's.

But if you look at the code above, notice that the selector that InDesign uses, audio.frame-5 has a class, and thus is more specific than a plain audio.

The answer lies in the HTML that InDesign creates. By default it generates an id element for each audio (and video) element that it creates, set to the filename. So, you can use that filename in the CSS to make your selector more specific, and thus override InDesign's CSS.

My audio file is called "bondia.mp3", and if you know CSS, you know there's a problem already. That period in the filename has special meaning in CSS (it means that what follows is a class name). Since we don't want that special meaning, we have to escape the period with a backslash:

audio#bondia\.mp3 {
width: 150px;

These values will now override the height and width values that InDesign generated. Save this text file as "extras.css"

How do I get them into my EPUB file? Go to Export in ID6, choose EPUB, click the Advanced panel, and then choose Add Style Sheet, then find your extras.css file. Export as usual.

EPUB Export Options

If you looked inside your EPUB file (though you don't have to), you'll see this:

Two CSS files

That's it! When you open your book in iBooks, you'll see that the audio controls are as you specified (and not as InDesign specified), even though you never had to deal with Terminal, never had to unzip or rezip or anything.

Fixed audio controls

The best part about this is that you can use this technique to override any of the CSS that InDesign either creates in a way that you don't like or that it doesn't yet support. For example, suppose you want to add a border around a video. You can do that with an extras.css file. (Indeed, you can add borders to all of your videos just by using video as a selector instead of targeting just a single video.) Suppose you want to add media-queries so that your document will work as best as it can in different ereaders. Just put the media-queries in your extras.css file. The possibilities are really enormous.

Thanks to Adobe for incorporating this amazing new feature!!


  1. What's the big deal with zipping and unzipping?

  2. @homer : well, sometimes, if you zip and unzip, some of the files are corrupted and you can start the process again :)

  3. That happens just about never. What operating system are you using?

  4. This is great, Liz. It sounds like you could completely define the elements of your ebook in a custom CSS file, then let InDesign handle all the rest of the mechanics of assembling the package. That would also make it a lot easier to maintain a common source file for print and ebook, because your InDesign style options and layout wouldn't have to accommodate both.

  5. Really Nice.
    Do we need to have the dame name between style indd and CSS?

  6. Liz, different question about InDesign - I am just starting with your books. Would you recommend trying a fixed-width layout eBook in InDesign? Or should that only be done in a native ePub editor?


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