With a bit of luck, you might be able to register your own bindings. On the other hand, since most reading systems are indeed built on top of rendering engines like WebKit, if they allows you to run Javascript, you can indeed do some 'reverse engineering' and try to fish the correct global objects they maintain and the document model they use for displaying. event bindings that can be used by JS code inside the EPUB to react to changes e.g. The EPUB 3 specification introduced a epubReadingSystem object for those reading systems that support Javascript, but it does not require that it should provide e.g. Please note that Javascript support is not present in EPUB 2, and only optional in EPUB 3, and many reading systems restrict the type of Javascript code you can have inside your EPUB. The reason is that the EPUB specification does not provide an official way for the document to 'know' the properties of the context - e.g., the current reading settings, like font size - of the application rendering it. There is no reliable way of doing what you want, where 'reliable' means 'working on any possible EPUB reading system'.

I tried to solve this by doing a fixed EPUB, but iBooks and ebook-viewer allow the font increase. The issue is that when the user changes the font, the calculation has to be done again. What I am trying to solve is that in spanish poetry when a verse is larger that the box's size, it has to be a break and the next line has to be right aligned with a bracket: I know about the epubReadingSystem and its limitations. So I suppose it is possible only because I can see trough the inspector of ebook-viewer. That is why I tried to detect the viewport scale with this or this. I see that when 1:1 changes, it's some sort of scale in int numbers. In ebook-viewer (aka Calibre viewer) I can see in the inspector that every time I change the font size the console prints Viewport cfi: /2/4/2/2/1:1. A listener that measures the scale of the viewport.

A timing event that measures the font size.I am trying to use or create and event listener when the user changes the font size of the EPUB in an ereader with Javascript but I haven't find the way.

Calibre font sizes epub