Ooooh, this is a good point, finally
http://www.idpf.org/epub/20/spec/OPS...m#Section1.3.5
Quote:
1.3.5: Relationship to CSS
This specification defines a style language based on CSS 2. [] A conforming Reading System must render all OPS CSS 2.0 required subset properties.
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http://www.idpf.org/epub/20/spec/OPS...htm#Section3.0
Quote:
3.0: OPS Style Sheets
[]
3.3: Properties
Default values for all supported CSS properties are as listed in CSS2.
The following table lists all CSS properties and values supported by this specification. Where not all values given in the CSS2 specification are listed for a given property, those values not listed are not supported by this specification. The column “Alternate display” indicates acceptable fallback display for CSS values that a Reading System cannot display as intended.
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and "position" is not listed, so it's not required by OPS. *But*, since it's not required, you *may* support it, and as far as I can see, the viewer of Calibre acts differently if the position is static or relative. So the viewer supports "position" property and, if so, you have to look at CSS2 specs, and I do not think that the viewer's behaviour is conformant to it.
So the viewer have to be conformant to CSS2 for the "position" property, or it should not support it at all, considering "position:relative" as "position:static". This could be a quick and easy solution, but IMHO it's not a good solution.