Default values in px and zoom are conflicting

[qnga]

Describe the bug

Some Readium CSS variables have default values in px in case they're not set. This is the case, for instance, of --RS__defaultLineLength. As this property is translated into a CSS property whose values must be in the zoomed coordinated system, setting a --USER__fontSize different of 1 leads to inconsistent (and bad) effective values.

How to reproduce?

1. Use the scroll mode.
2. Do not set --RS__defaultLineLength or --USER__lineLength so that the default value applies.
3. Change --USER__fontSize to 0.1. If your viewport is large enough, you'll see margins far bigger than with --USER__fontSize == 1 because line length is limited to 640px in the zoomed coordinate system instead of the original one.

Readium CSS version

2.0.0-beta.13

Platforms

Android

Browsers

Android System WebView

Additional context

No response

[JayPanoz]

Note to self.

In practice I'll have to review any unit impacted by zoom, so that includes vh, vw, etc. In body scope. We know from Thorium Desktop's experience they are being scaled.

I can think of safeguard for img already.

But of course these are just the values from ReadiumCSS, the ones from authors are another issue entirely.