[Implementers’ doc] [Authors’ doc]
Source: EPUB Content Documents 3.2
There’s a few criteria Reading Systems must meet in order to be conformant:
viewportmeta tag to render XHTML Content Documents;
heightattributes in SVG to render SVG Content Documents.
You should not inject additional content such as borders, margins, headers or footers into the viewport or the appplication area surrounding the viewport.
When the aspect ratio does not match the aspect ratio of the Reading System, you may position the containing block inside the area to accommodate the user interface (letter-boxing).
Content positioned outside the containing block will not be visible.
SVG is a little bit more complex as there are different options to take into account.
If the SVG only has a
viewbox attribute, the coordinate system it defines is mapped to the viewport, keeping the aspect ratio.
If the pixel values defined by
width exceed the viewport’s pixel values, the graphic must not be rescaled (it must be clipped on the viewport boundaries).
The coordinate system defined by the
viewbox is mapped to the viewport, keeping the aspect ratio.
rendition:spread portrait and
rendition:viewport are deprecated in EPUB 3.2.
rendition:spread-portrait spine override is deprecated as well.