[dgatwood]

If it bothers you enough, file a Radar with Apple asking for iBooks to include its own copy of WebKit so that its behavior won't depend on what OS it is running on.

That said, if there's a difference in behavior, that probably means that iBooks is setting some magic -webkit- properly that is supported on iOS 6 and not on iOS 5. So you can probably make them consistent by changing that property. Then you'll still have to tweak it to make iOS 5 look right, but at least you won't have to do extra work to keep iOS 6 from breaking while you're doing it.

Some good WebKit properties to define include:

html {
        -webkit-line-box-contain: block inline replaced !important;
        line-box-contain: block inline replaced !important;
        -webkit-nbsp-mode: normal;
}

html body {
        -webkit-hyphens: auto; /* if you want auto-hyphenation, else manual */
        -webkit-line-box-contain: block inline replaced !important;
        line-box-contain: block inline replaced !important;
        -webkit-font-smoothing: antialiased;
}

p {
        -webkit-line-box-contain: block inline replaced !important;
        line-box-contain: block inline replaced !important;
}

I recommend taking a look at the UA stylesheet for iBooks. It's fairly easy to do.

1. Run iTunes.
2. Right-click (or control-click) on iBooks and choose "Show in Finder".
3. Make a copy of the IPA file in another location.
4. Rename the copy to end in .zip.
5. Double-click the ZIP file.
6. Right-click the resulting IPA package and choose "Show Package Contents".
7. Double-click "Payload".
8. Right-click the iBooks.app bundle and choose "Show Package Contents".
9. Examine the CSS files:
   • AEAssetShare.css
   • user_stylesheet_base.css.tmpl
   • user_stylesheet_flowable.css.tmpl (this is the important one)
   • user_stylesheet_picturebook.css.tmpl