Using percentage instead of pixels

I know this is going kind of deep into the ACBF format, and may be too late already to change, but there is an issue with ACBF if you re-size the images used for the comic at a later point, as all the frame and speech bubble regions are defined in pixels, and there is nowhere in the ACBF file that tells you the size of the original images, so if at a later point you replace them with higher or lower resolution pages, the frames will be in the wrong place.

Comics creators will usually work in much higher resolution than what they would distribute for download, typically something like 10"x7" at 600dpi, or 6000x4200 pixels, and will then export lower resolution JPEG's for distribution. If you use ACBF to define your frames and speech bubbles, you'll be stuck at using whatever resolution you originally exported your comic in, or having to go through and redefine all the panels again.

Is there any way to support percentages, or some kind of resolution independent frame and speech bubble definitions?

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The reason I'm asking is mainly because I'm working on a simple JavaScript web comics reader that supports panel-by-panel reading (code is here https://github.com/ragtag/panel-by-panel). It's main goal is to create a simple to use tool for web comics artists to make their web comics mobile friendly. I am currently using a basic JSON format for defining the pages and panels, and using percentages instead of pixels (x,y,width and height of each panels in %). As far as I can tell the EPUB3 format (http://www.idpf.org/epub/renditions/region-nav/#sec-3) seems to have gone with percentages too, though I don't know of any tool that can either make or make use of that data.

I haven't written an interface for defining the pages and panels yet, and figured it would be better to rely on existing standards, such as ACBF, rather than trying to re-invent the wheel. My current plan is to do one or more of the following:

1. Write a simple command line script that converts ACBF data into a json that works with my reader.
2. Writing support directly in the JavaScript reader that uses ACBF xml files directly.
3. Writing an exporter for my Gimp Book plug-in to either my format or ACBF (Wolthera has already implemented most of ACBF in her Krita Comics Manager).
4. Write my own little tool for defining panels (re-inventing the wheel).
5. Something clever, I haven't thought of yet. :)

As there are already quite a few comics in the ACBF format, it makes sense for me to implement some kind of support for it in my reader. For 1 and 2, changing image resolution would be an issue, 3 would solve this, as you would be going from the original to a scaled version, as you genearte the ACBF xml.

I think I've gone a bit of track here, and it's late, but thoughts and input are welcome. :)